I often think about this. My philosophies and outlook on life are a quantum leap from those of the people who surrounded me. I resent going to a rubbish school, I resent having alcoholic father, I resent having no real positive influence on me. Such is that I only began properly living when I got to 30. Before that I lived a nothingness life, in the pub, earning beer tokens, not having holidays or driving etc. I never knew people who spoke about holidays and sport etc.
I've have loads of advice, but if you pin me to 1 per section, it'd be:-
When you were starting senior school - don't be embarrassed to follow a talent. There's nothing "gay" about being musically talented or athletic, do not let those who don't have talent make you bury your own talent for fear of being ostracised. When you're an adult, most of those people you go to school with won't matter in your life.
Then another piece when you turned 21 - try and secure a career that doesn't have a limited amount of growth, Don't go into something like caring, no matter how noble, it will never pay. What decisions you make career wise (and pensions/mortgages etc) will sort you out when you get to 40s.
Then one final piece when you turn 40 - I'd like to think most people won't have many decisions that they need to make at 40. I would say, be as fit and active as you can, try and do something at least 4 times a week.