You're disputing a contingent aspect of the analogy that isn't related to the problem with the OKCupid article.

To establish that "older people get tested more -> older people have less undetected disease", you first have to establish that older people started out with the same base rate of disease as younger people. The OKCupid article doesn't do that, and knb is correctly calling them out on it.

I agree that younger people in fact have higher STD rates than older people, but OKCupid makes a fallacious argument for this, and it is acceptable in philosophy to criticize fallacious arguments for true conclusions.

Note that I specifically said above that the OKCupid essay's "conclusion is correct." You're confusing criticizing an argument form and criticizing a conclusion. Consider the following exchange:

Person A: Karl Marx was an evil person, therefore communism is wrong.

Person B: That's an example of the ad hominem fallacy, and so an invalid argument.

Person A: How dare you say communism wasn't wrong, you Stalinist pig!

Posting statistics about exactly how many people were killed in communist regimes does not make A's argument valid, or B's criticism wrong.

I'm no expert, but I think the STDs to be most concerned with those are those which are least treatable. Wikipedia says the ones you cite are bacterial, so I presume, completely destroyed by antibiotics.

So, viruses like HIV and herpes? And maybe HPV?

(still, perhaps the treatable STDs have already done some lasting damage by the time they're detected, anti-bacterials may be harmful, and it's still gross, so use condoms)