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.
Well, that's just nitpicking. The implicit assumption (that young women aren't much less likely a priori than old women to have STDs) is so uncontroversial that it's not such a fatal mistake to not make it explicit. For that matter, it doesn't make explicit the assumption that ceteris paribus people would prefer not to have STDs, either.
Why don’t men go for younger women? That’s not quite accurate, because they do, but we seem to have a cultural norm against girls who have recently exited puberty, even though they are highly fertile. Some time in the last few hundred years, we reached a cultural norm that as a man gets older, it’s increasingly less appropriate for him to be with a sixteen year old. That’s not long enough to override thousands of years of evolution, so what could have contributed to it?
The best solution I’ve heard started by looking at who benefits from this norm [older women] and wondering whether they could have contributed to it. After all, the strength of this norm has been increased in the last sixty years or so, which coincides with the period in which women’s power has increased.
One alternative I heard recently is that it doesn’t make sense biologically to go for women who are recently post-pubescent, as they don’t make the best mothers. Instead, a slightly older woman [mid-twenties] makes sense to be the best mother. This is perfectly plausible.
Another, less plausible, suggestion I’ve heard is that it’s to do with mental capacity. I find this unconvincing because we have few objections to a high-status man dating a beautiful but low-intellect woman.
Thoughts?