I trust your mother (my sister) will have the good sense not to share this with you until you've reached a sufficient level of maturity. I also trust that you will both eventually reach a sufficient level of maturity.
Right off the bat, I should probably say that this letter isn't motivated by any sort of existential crisis. I've been pondering my own mortality for a while now, but I'm not planning to leap into the welcoming embrace of the grave any time soon. I hope to be around long enough to see you dismiss my advice as the fevered rantings of your ne'er do well uncle, which is as sensible a response as any, considering my own considerable shortcomings.
Anyway, here are a few things I think I've learned:
1. Safety is overrated.
2. Financial security is a wonderful thing, but not at the expense of your happiness.
3. I'm not a big fan of the notion that your parents should be your friends, but in your case, I'll make an exception, only because you guys kinda hit the parental lottery. Your parents aren't just good people, they're both smart and funny and adaptable. That's a rare combination. They've pulled my ass out of the fire more times than I care to remember, and they've both cheerfully wiped the slate clean afterward. You couldn't ask for better friends.
4. Do not dance for money, clothed or otherwise. And if you're thinking about ballet, I've got two words for you: "Black Swan".
5. Don't prejudge people. That said, once you feel you've got a firm grasp of the facts, don't be afraid to post-judge.
6. As an addendum to #5, never forget that people can change. By all means, protect yourself from users and haters, but leave a little emotional investment in the bank. They may surprise you.
7. Don't ever call America "the greatest nation on Earth" until you've seen a few other nations.
8. When it comes to race, don't be "colorblind". There's no problem with seeing color. In a bland, sometimes frustratingly conformist world, color is a thing to be treasured.
9. Don't make the same mistakes I did. Make your own damn mistakes.
10. Help people when you can, but when a buffet is laid out before you, help yourself.
11. If you must join a "swing choir", fine. But never mistake it for rock and roll.
12. Never stop playing the music too loud.
13. Take a lot of pictures.
14. Be cynical and sarcastic, but be informed. And don't be bitter.
15. The Rolling Stones are the world's greatest rock and roll band.
16. Be empathetic, but don't be maudlin.
17. Explore religion, but keep your money. Any God worth worshipping ain't broke.
18. Bad things happen to good people. And good people get over it.
19. Don't let fear make your decisions for you.
20. Finally, and this may seem to contradict some of my previous advice, but so what? What doesn't kill you doesn't necessarily make you stronger. Sometimes it just hurts, and those are the times you'll need strong folks around you. People are work, but when the chips are down, the work can pay off in spectacularly comforting fashion.
Twenty seems like a nice round number, so I'll stop there. We've only really known each other for a relatively short time, but it seems to me that you're both pretty far ahead of the game already, intellectually speaking, and what's more, you're both pretty fun to be around. And that's coming from a guy who can't stand kids.
Have fun with your lives, Mae and Gwen, and try to do some good along the way. And if either of you hit the big time, buy a beach house for your uncle.
Sincerely, your biggest fan,