If it makes you feel any better, as someone in his late 20s that has led a couple teams and is eyeing management, I can't think of any particular reason not to hire you (except maybe that you'd demand more money than a recently started programmer, but now we're just talking cost-benefit analysis, which seems a fine basis for decisions) and several benefits, assuming you'd be okay taking direction from someone that much younger than you.

Can you perhaps elaborate on the kind of responses you get or conjecture on why you think people are making those kinds of choices?

I've heard similar stories from mentors and older friends who work in tech, but I've never really gotten a chance to discuss it in detail. It's kind of an awkward topic, so I won't take it badly if you decline, I'm just honestly hoping to hear a view that I'm not exposed to, being young and fresh in the market.

It's not universal, on my team of 8 there is a ~60, ~65, ~70. I use ~ because I have never actually asked, and that's unlikely to change.