I just turned 20... what is the most valuable lesson you have learned in your life so far?

Invest early, start now, wealth creation is usually a long term game. Many people wait until they are older (if at all) but start compounding now if you have not done so already. That applies to everything else in life - compound forward all that you do. If you keep changing course you will lose momentum.

Alcohol is insiduous and can easily steal a lot of time from you. I lost my 20's to alcohol and other substances.

Learn who you are, craft who you want to be. Set your values and live by them.

Find good mentors.

If you think you might want to start a family, consider this sooner rather than later.

Keep yourself emotionally in-check, be kind to yourself and to other people. Be progressive in attitude. Respond to difficult situations that present themselves, do not react with the first emotional response.

When I was 20 I thought I had a whole lifetime ahead, and, and I did and so do you. But time is precious, we have so little of it and it really does pass quickly. Use your time wisely.

I will tell you this at age 58,

1. you can't even guess where your life will take you but you can try to keep it heading in a certain direction of your choosing.
2. No matter what job you have, and you will likely have a number of them, make sure to learn something from all of them no matter what they are.
3. There is a much older you that doesn't exist yet and he would greatly appreciate it if you save a single dollar today for him because when he gets it he will be able to buy $50 worth of stuff with it. Save for retirement now no matter if it's $10/month, put it where you can not get it back unless it's a tremendous emergency.
   3(b). Almost everyone I know blew every dollar they made till about age 30, the ones that didn't do that got a jump on the rest of us that lasted their whole lives.
4. Take care of your health now, it will make that old man healthier as well and he will thank you for it. I never did brutal sports and injured my neck, back, or knees when I was young. The people I know who did are suffering now and will continue for the rest of their lives. Hip replacements, knee replacements, rotator cuff surgery - none of those things replace having the original equipment in good working order.
5. Eat well - it directly impacts your mental horsepower
6. Sleep naturally -if you have to set an alarm to get up then you aren't going to bed early enough. Let your body rest until it's rested.
7. Mental health is as real as physical health - don't fill your head with porno, endless wasted hours of violent video games, pop culture bs, crap tv series, etc. Don't grind, don't try to control what you can't, don't worry yourself sick. Take decisive action rather than worry. Try reading interesting, informative, thought-provoking books instead.
8. Have plans, time flies so fast you will not believe it, you can change your plans 100 times if you want but living without any plan at all is like wasting time because you arent going anywhere.
9. Don't be a doormat for the world, you matter, what you want has to be included in considerations, you get a vote, your needs are legit, you have a say. Don't let anyone assume you are cool with things the way they want them without bothering to check with you.
10. The opposite of that - be generous, loyal, patient, caring, and kind even when you know it may go unnoticed or unrewarded.

Bonus point #11 - people my age die from disease, people your age die from stupidity - nothing good happens after midnight, don't be a DUI a-holes victim, they always survive, you won't. Getting robbed, drugged, assaulted, etc. 90% of that has to do with you being where you shouldn't when you shouldn't. Hang out with bad people, go to bad places and you will eventually have a bad problem come and find you.

In school you were taught to trust what you read. Do not do that on the internet. Do your research and make your own decisions.

For example, read people's first contact with iPod in 2001:

Apple releases iPod.
"No wireless. Less space than a nomad. Lame."

Or discussions when Bitcoin reached 1 dollar parity in 2011:

Online-Only Currency BitCoin Reaches Dollar Parity
"Anyway, another fad "currency". Might as well collect "ISK in eveonline" or "gold nuggets in WoW" - same thing."

So when today in 2021 you read people saying with confidence that something "will never work", take it with a huge grain of salt.

My lesson that I'm gonna share with you is about empathy:

"Every person has a reason for doing what they are doing. It's not just completely senseless or irrational behaviour like I generally assumed simply because I wasn't able to empathize myself into the thoughts and drivers of other people ."