A Quora recently asked: what do 50-year-olds know that most 20-year-old don't? This answer by Franklin Veaux explains.
1) When your heart breaks, you will feel like you will never love again, and you will be alone forever and ever. That feeling is a lie. The night passes. Dawn comes. You will be fine.
2) When you meet a stranger and your eyes lock across a crowded room, and every fiber of your heart and soul sings "Yes! Yes! This is the one! I am in love!" you are not in love. That's the very old part of your brain saying "Our offspring would have awesome immune systems." You can not love someone you do not know. When you feel that feeling a couple of times, you'll learn to recognize it for what it is.
3) Movie stars do not generally make good science analysts, political thinkers, or life coaches. If you take medical advice from former Playboy centerfolds instead of doctors, or think that B-list movie actors are better sources of political ideas than actual political scholars, you're apt to end up somewhere you really don't want to be.
4) All the angst and turmoil and alienation you feel is as old as written history. You aren't the first to feel these things and you won't be the last. All of this has happened before, and all of this will happen again.
5) Incremental progress toward your goal is better than stomping your foot and saying "if I can't have everything I want right now, I'm going to burn it all down!" Incremental progress is progress. It is impossible for dead Utopians to build Utopia.
6) The world changes faster than you think it does. I was born into a world where everyone believed the eternal struggle between the Soviet Union and the Americans would destroy mankind. Being gay was still illegal in many states when I was born. Now gays and lesbians can marry, and there are voters who weren't alive when the USSR was a thing. See reference to "incremental progress is still progress" above.
7) The things you consider life or death matters of supreme importance now, you might not even remember, much less care about, thirty years from now. Choose your battles wisely.
8) It takes decades to develop a reputation for integrity and trustworthiness, and just one single decision to destroy it.
9) Other people are real. They matter just as much as you do.
10) Nothing is as simple as you think it is, even when you factor in nothing is as simple as you think it is.
11) Small decisions have big consequences down the road. Imagine a space probe on its way to Mars. The tiniest, tiniest course correction at the beginning of the trip can mean making it there or missing by millions of miles. Every time you're scared to do what you want to do, but you do it anyway, in any matter however small, you're making a tiny course correction. Added up over time, they make a huge difference in your life and take you somewhere awesome.
12) Life rewards courage.
13) If you're ever stuck in a position where you have to make a choice but you don't know the right thing to do, the right thing to do is usually the thing that takes the most courage.