I binge drank between 14-16 years old, started smoking weed around the same time - and smoked daily from 20-25. After that I did a short bout of therapy and walked away from it. I'm now 40 and I do often wonder what damage might have been done.
If I'd have to guess how damage could materialise i'd point to the fact that my thinking was polarised for a long time and I had trouble controlling my emotions.
But those traits could easily be attributed to verbal aggression and a lack of trust and empathy in my childhood - which most likely contributed to the substance abuse in the first place (1) Or it could even just be attributed to the male brain taking a little more time to mature.
(1) https://www.theguardian.com/society/2023/oct/02/shouting-at-...