Using during those years, 12/13-early twenties, is one of the easiest ways for the brain to develop major mental illnesses as a result.

What? Source please because I'm really curious where you're getting this from. Please send me the studies that show that drug use causes "major mental illness." If that was true, many people would have major mental illnesses.

Addiction and mental illness are often comorbid but,

"The high prevalence of comorbidity between drug use disorders and other mental illnesses does not mean that one caused the other, even if one appeared first. In fact, establishing causality or directionality is difficult for several reasons."

Which came first, the chicken or the egg?

Obviously yes, drugs can exacerbate mental illness and can cause psychotic symptoms in some people, but I'm really curious where you got the information that drug use in adolescence is one of the "easiest way to develop mental illness."

And mental illness does not equal brain damage? Are we talking about mental illness or brain damage here? Because OP asked about brain damage

You don't seem very educated on this topic and telling someone their "brain is fucked up and it's never gonna get better" is not just untrue but unhelpful.