Its possible that I was born after the creation of your account.
sizeable minority maybe like 10%-30%, i've seen many posts about Python/ML, i'm very convinced most of the people who focus greatly on Python to do ML are under 30. No source to back it up, just logic - python exploded in the past 5 years and it's hand-in-hand with ml...college kids must be loving it.
ML is currently the hotness in tech, and startup-land in particular, so it's a popular post topic here as well. (At its foundation this is a startup forum - it's owned by the silicon valley venture capital firm Y Combinator, hence the url.)
You might be right that younger people are particularly interested in AI, but if I had to guess I would say HN skews older than other forums like Reddit.