I wouldn't assume that it did. I would focus on its remaining extreme plasticity. It seems likely that you could do an enormous amount of self-cultivation via practices like meditation.
Cannabis absolutely affects brain development if you use it consistently throughout adolescence. Don't get me wrong, I still favor legalization and decriminalization, but the number of people who like to pretend like it's some super happy wonder drug with no downsides whatsoever bother me. Like any drug, there are side effects, and this is one of them.