First off, I didn’t properly appreciate being young, because I didn’t know any other way to be. I’m pushing 40 now, and things are different. The knees are a little creakier, it’s harder to just roll out of bed in the morning and leap headlong into my day. I was able to eat any old crap food and still feel fine. I could stay up all night and function relatively well the next day. These are things you lose as you get older, so make the best of them now, and appreciate having them. I wish I had.
The next two things are kind of a contradiction. On one hand, I wish I had prepared earlier for the inevitability of responsibility. What I mean is, I wish I had worked harder in school (so I wouldn’t still be working on an Associate’s degree at 39), I wish I had started saving for retirement earlier (because the sooner you start putting in, the more money you make through the magic of compound interest), and essentially, setting up my life. I thought that’s just what “adults” did, not realizing I was a young adult.
On the other hand, I wish I had done more crazy shit. More spontaneous road trips at 3am, just to get somewhere where it would be cool to see the sun rise. More adventures. More life experiences, some of which seem miserable at the time (like taking off for a strange city with my then-boyfriend, and couch surfing with people he remembered from college, living only on a loaf of sourdough and a gallon of chocolate milk for the weekend – see “eating any old crap,” above) but which provide memories for the rest of your life. You eventually have to make some kind of living, add responsibilities, live the work-day drudge, and when you’re young is when you have the fewest ties and responsibilities, and you can more easily take off on some wild goose chase. I should have done a couple years in the Peace Corps, or something.
One more thing. You probably have people around you now, older people that you love, like parents, grandparents, family friends. They seem rock solid, like they’ll always be there. Something that surprised me as I got older (although it shouldn’t have) is that these people get older, too. They become frail, the die eventually. As a middle-aged adult, I wish I still had some of these people around, to ask for advice, to show them how I turned out as a grown-up. I’d just love to talk with them from an adult perspective for a day, which is something you don’t get if they pass on while you’re still young. So talk with them. Maybe even record them, or write down things about them, or things they say. They seem like they’ll be there forever, but they won’t be.
Those are the biggies. Mostly, it could be summed up as “Don’t let anyone else try to fit you in a mould. Learn who you are, before you don’t get a chance to do so anymore. At the same time, take care of yourself.”