The brain is not mature until the late 20's so why are we treating 18 yos who cant drink like citizens?

No wonder health care extends till age 27 and I even heard some time ago they were talking about raising age for child support till 30.The part of the brain to control judgement and impulse is not mature until the late 20's plus life expectancy is extending so why dont we raise the age of adulthood to like 35 since thats when a person is mature although a matter in your brain also grows in the 40's too.