The argument Robin Williams character is presenting is roughly that reading about experiences as a young person is less genuine than personally living through said experiences. Perhaps that there's more merit to a belief when it comes from someone who founded it through their own lived experience than a belief which comes from reading about others' experiences.

The irony is that Good Will Hunting (an outstanding film, FWIW) was written by Matt Damon and Ben Affleck when they were very young. The belief that experience is necessary to true understanding is perhaps advice they'd gotten from someone else, not an axiom they came to through _their_ experience.

It doesn't make Williams' monologue any less true, and I imagine 30y later Damon and Affleck they'd stand by the advice, but it does make for an interesting philosophical quirk.

I think it's ironic because the old man is schooling the kid, but the kid in that scene literally wrote the words for the old man.

Maybe irony is too strong, but it's amusing at least