No, it is not. I am 49 and I have no problems using HN. Also often it is clear, that some of the amazing answer found here, are from people way older than in the their 20s, because some of those answers requires the poster to have many many years of experience in order to come up with that answer.

There is a great mix of young and old here :)

The problem with HN is that it has too many content not related to tech, like lots of politics and also is too american focused. Anybody not from america gets a feeling of being a little weirded out by some of the things and morals discussed around here. At least for me that's a problem. If that dev.to gets nice content and doesn't get all these america specific morally political stuff going around maybe I'll use it.

HN is for everyone I think, not only devs, not only 20s ...