It obviously has 'any' validity. If an instance of 'ancient wisdom' killed off or weakened the followers enough, it wouldn't be around. Also, said thing has been optimized for a lot of time by a lot of people, and the version we receive probably isn't the best, but still one of the better versions.

While some will weaken the people a bit and stick around for sounding good, they generally are just ideas that worked well enough. The best argument for 'ancient wisdom' is that you can actually just check how it has effected the people using it. If it has good effects on them, it is likely a good idea. If it has negative effects, it is probably bad.

'Ancient wisdom' also includes a lot of ideas we really don't think of that way. Including math, science, language, etc. We start calling it things like 'ancient wisdom' (or tradition, or culture) if only certain traditions use it, which would mean it was less successful at convincing people, and less likely to be a truly good idea, but a lot of it will still be very good ideas.

By default, you should probably think that the reasons given are often wrong, but that the idea itself is in some way useful. (This can just be 'socially useful' though.) 'Alternative medicine' includes a lot of things that kind of work for specific problems, but people didn't figure out how to use in an incontrovertible manner. Some alternative medicines don't work for any likely problem, some are more likely to poison than help, but in general they solve real problems. In many cases, 'ancient wisdom' medicine is normal medicine. They had a lot of bad ideas over the millenia medically, but many aso clearly worked. 'Religion' includes a lot of things that are shown scientifically to improve the health, happiness, and wellbeing of adherents, but some strains of religion make them do crazy / evil things. You can't really make a blatant statement by the category.

When it comes to Buddhist practice, it's worth noting that practicing techniques by the book is not how Buddhism was practiced for most of the time in the last 2500 years. It was mostly an oral tradition and as such the knowledge that's passed down from teacher to student evolves over time in various ways. 

Many modern Buddhist tradition put much more emphasis on meditation in contrast to ritualized behavior. 

In Buddhism (and in Christanity for that matter) for thousands of years meditation was largely done in monasteries and not by lay-people. In many Buddhist communities "lay-people aren't supposed to meditate" is something you could call "ancient wisdom". 

In someone convinces you in a Western context that following some practice is ancient wisdom, they are likely doing a lot of picking and choosing in a way that does not make it clear how ancient the thing they are promoting actually happens to be. 

I'd read Nassim Nicholas Taleb on the Lindy effect for the strongest defense of this proposition. Basically all ideas and culture are constantly fighting in a market of culture. For every Jhana there are 1000 different spiritual concepts, which try to occupy the same niche. There has to be something to Jhana's that leads to it still being done today, while the rest became history. That something does not have to necessarily mean that the idea is true, for example meditation in general is known to be very good for you, so if Jhana's work as the carrot on the stick to get people to meditate, then the idea would also stick around, as people start praising Jhana's due to the benefits they got from meditation. But every idea that is old needs to have some sort of payload, something that helped it survive on the market of ideas and culture for millenia.

I would tend to give particular credence to any practice which pre-dates the printing press.

The reason is fairly straightforward. Spreading ideas was significantly more expensive, and often could only occur to the extent that holding the ideas made the carriers better adapted to their environments.

As the cost to spread an idea has become cheaper, humans can unfortunately afford to spread a great deal more pleasant (feel free to substitute reward hacking for pleasant) junk.

That doesn't mean failing to examine the ideas critically, but there are more than a few ideas that I once doubted the wisdom of, which made a great deal more sense from this perspective.

As for the particular practice of meditation that you reference, I tend to view spiritual practices as somewhat difficult to analyze for this purpose, as the entire structure of the religion was what was transmitted, not only the particularly adaptive information. To use DNA as an analogy, it's difficult to tell, which portions are of particularly high utility, analogous to the A, C, G, and T in DNA, and which serve as the sugar-phosphate backbone. Potentially useful in maintenance of the structure as a whole, but perhaps not of particular use when translated outside that context.

Which portions of Buddhism are which, I couldn't tell you, I lack practice in the meditation methods mentioned, and lack deeper familiarity with the relevant social and historical context.

Ancient wisdom is not scientific, and it might even be false, but the benefits are very real, and these benefits sort of works to make the wisdom true.

The best example I can give is placebo, the belief that something is true helps make it true, so even if it's not true, you get the benefits of it being true. The special trait ancient wisdom has is this: The outcome is influenced by your belief in the outcome. This tends to be true for psychological things, and advice like "Belief can move mountains" is entirely true in the psychological realm. But scientific people, who deal with reality, tend to reject all of this and consider it as nonsense, as the problems they're used to aren't influenced by belief.

Another case in which belief matters includes treating things with weight/respect/sacredness/divinity. These things are just human constructs, but they have very real benefits. Of course, you can be an obnoxious atheist and break these illusions all you want, but the consequences of doing this will be nihilism. Why? Because treating things as if they have weight is what gives them weight, and nihilism is basically the lack of perceived weight. There's nothing objectively valid about filial piety, but it does have benefits, and acting as if it's something special makes it so.

Ancient wisdom often gets the conclusions right, but get the explanations wrong, and this is likely in order to make people take the conclusions seriously. Meditation has been shown to be good for you. Are you feeling "Ki" or does your body just feel warm when you concentrate on it? Do you become "one with everything" or does your perception just discard duality for a moment? Do you "meet god" or do you merely experience a peace of mind as you let go of resistance? The true answer is the boring one, but the fantastical explanation helps make these ideas more contagious, and it's likely that the false explanations have stuck around because they're stronger memetically.

Ancient wisdom has one advantage that modern science does not: It can deal with things which are beyond our understanding. The opposite is dangerous: If you reject something just because you don't understand why it might be good (or because the people who like it aren't intellectual enough to defend it), then you're being rational in the map rather than in the territory. Maybe the thing you're dismissing is actually good for reasons that we won't understand for another 20 years.

You can compare this with money, money is "real but not real" in a similar way. And this all generalizes far beyond my examples, but the main benefits are found, like I said, in everything human (psychological and spiritual) and in areas in which the consensus has an incomplete map. I belive that nature has its own intelligence in a way, and that we tend to underestimate it.

Edit: Downvotes came fast. Surely I wrote enough that I've made it very easy to attack my position? This topic is interesting and holds a lot of utility, so feel free to reply. 

Survival of a meme for a long time is a weak evidence of its truth. It's not zero evidence, because true memes have advantage over false ones, but neither it's particularly strong evidence, because there are other reasons for meme virulence instead of truth, so the signal to noise ratio is not that great.

You should, of course, remember that Argument Screens Off Authority. If something is true there have to be some object level arguments in favor of it, instead of just vague meta-reasoning about "Anscient Wisdom". 

If all the arguments for a particular thing are appeals to tradition, if you actually look into the matter and it turns out that even the most passionate supporters of it do not have anything object-level to back up their beliefs, if the idea has to shroud itself in ancestry and mystery, lest it will lack any substance, then it's a stronger evidence that the meme is false.

Claims about "if you keep doing this thing, after a lot of hard work you will achieve these amazing results" seem memetically useful regardless of their truth value. It gives people motivation to join the group and work harder; and whenever someone complains about working hard but not getting the advertised results, you can dismiss them as doing it wrong, or not working hard enough.

Also, consider the status incentives. Claiming to achieve the results after a lot of hard work is high-status; admitting to not achieving the results is low-status; and the claims are externally unverifiable anyway.

I believe monks have a taboo against talking about their attainments

I suspect this rule appeared as a consequence of many monks following the status incentives too obviously. Letting them continue doing so would be good for them but bad for the group, so the groups that made the taboo were more successful.

(Cynically speaking, the actual rule seems to be: Low-status people are not allowed to talk about their attainments. If you are high-status, others will make assumptions about your attainments, and you can just smile mysteriously and speak some generic wise words, or otherwise confirm it in a plausibly deniable way.)

Age and popularity of an idea or practice have some predictive power as to how useful it has been.  Old and surviving is some evidence.  Popular is some evidence.  Old and NOT popular is conflicting evidence - it's useful (or at least not very harmful) to some, perhaps limited by context or covariant factors that don't apply elsewhere.  

Whether your interpretation of a practice will get benefits for you should probably be determined by more specific analysis than "it worked for a small set of people in a very different environment, and never caught on universally".

Some stuff just works but for reasons unknow to the practioner. Trail and error is a very powerful tool if used over over many generations to "solve" a particular problem. But that do not mean anyone know WHY it works.

It seems plausible to me that there is a sort of selection process in which people are creating ostensible-wisdom all the time, but only some of that wisdom gets passed along to the next generation, and the next, and so forth, while a lot of it gets discarded. If some example of wisdom is indeed ancient, then you can by virtue of that have at least some evidence that it has passed through this selection process.

To what extent this selection process selects for wisdom that actually earns that designation I'll leave as an exercise for the reader.