Don’t worship people or companies, Like them or their work, But be ready to move on. This will save you a lot of dissapointment and will also open you up to trying more things instead of sticking to the one.
Limit your drinking and don’t get fat.
Just because somebody is older or in a position of authority doesnt mean they are right. (Aimed at the careers advisor at school who told me to forget about being a coder as computers are a passing fad)
Don’t be scared to limit (or stop if you are in the position) your contact with people who are toxic to you, That includes parents and siblings.
I’m sure I will think of loads more but it’s time to eat :)
More generally, the grownups don’t know better than you. They are just repeating what they’ve been told by someone else who also didn’t know any better. More to the point, adult life is like high school, only more so.