There's a lot of evidence that most people mellow out with age, age-crime curves show this clearly with dramatic peaks in the late teens. Teenagers and young adults are more prone to acting impulsively, with less foresight for the future.
There may be some people who are irreparably 'genetically damaged' from birth and will never improve, but this isn't the general case.
I feel like most of us kind of intuitively know this at a somewhat visceral level. If a two year old does something obnoxious (e.g. throws food at you), I think most people won't be nearly as upset as if a 40 year old did. I know when my nieces or nephews do something annoying, it's substantially easier for me to roll my eyes and say something like "they're just kids, don't get angry with them" than it is when a full-on grownup annoys me.
I'm quite confident that 18 year old me was still basically a child. I never committed any violent crimes when I was 18, but I did a lot of other dumb irrational things that I don't think I'd do now.
There may be some people who are irreparably 'genetically damaged' from birth and will never improve, but this isn't the general case.