I think it's probably just VC bias driven by the number of unicorns which had young founders (VCs aren't after 'moderately successful startups', where founders tend to be older, they're after wildly successful unicorns).

Those young founders hire "people like themselves" which naturally means people their age or younger.

There's a certain degree of implicit age discrimination baked into the default hiring process of most startups too - all of those questions about reversing binary trees or implementing quicksort biases hiring toward recent grads who have just learned it and away from the experienced devs (because 99% of devs simply never use this kind of theory).

tl;dr VCs cargo cult and end up investing in young founders, young founders cargo cult their interview process and hire people "like themselves".

That hasn't completely escaped, but with a family and 25 years experience, I put that into a different perspective now.