48 here. Quite happy really. I feel I had a LOOOOONG life already. Born in france, got in computers at 12, programmed my life away until my 20's -- did projects at 18 that most people would dream of even now -worked for Apple when it wasn't cool, take that you hipster beepstards- , started playing guitar in bars in Paris clubs at night, and working during the day; drank a lot, clubbed a lot (mostly music clubs); then moved to the riviera for a few years. Work, Sun, etc etc; then I moved to the UK 18 years ago now.
Been working a lot, lots of cool companies, projects; I'm married with my soul mate (met her when I was 21), we don't have kids (we hate kids) -- we earn quite a lot, each; we own 3 houses, some land for stomping around.
So yeah, all the old little injuries you picked when you were <20 are waking up to remind you they weren't entirely gone -- needs to be careful about drink, and general health, picked up the 'hereditary' high blood pressure stuff -- luckily I've calmed down a LOT since my 20's so it's manageable...
But overall, I had a pretty good ride so far, I wish people didn't 'worry' as soon as they are 25 about pensions, houses and all that ****, and actually lived their *own* lifes, not their parents or their kids; that way they wouldn't feel all bitter and twisted when they hit 40.
But, the ONLY THING people should know is to make themselves 'sellable' : employable, sharp. That way they can make money, and ultimately that will buy you time. Everytime you do anything consider wether it's a good investment of your time; TIME is the precious thing you'll miss later on, especially if you have kids who will just suck all your time away. I seriously wish days were 30 hours so I could do MORE stuff I like to do!