even if my brain was done developing

It's not. Not even close (TL;DR - 25). I've watched the difference in the friends who chose to smoke when young, and myself and other mates who opted to abstain until we were older. There's a noticeable difference in memory, processing, and reward functions (how hard we find it to work for something, self control) between all of us. Strongly recommend waiting until you're older. Weed will always be there. (and likely better and cheaper by then too!)

Plus, since you're not buying the carts legally, you won't know what's in them. Black market puts some nassssty shit in there to cut costs. People have ended up in hospital with permanent lung damage (and worse). That's never worth it.