Every man, by the time they're 20? Well shit, I'm almost 20! Here's my list:
Have a basic understanding of major vehicular systems. As a mechanic, I see people nearly every week who don't understand how to install a spare tire, or why you need to change your oil every 5,000 Km, or what lug nut torque is or why it's important. You may not want to work on your vehicle yourself, but at least understand why your tie rod needs replacing or what is involved with changing that "cat" your mechanic keeps telling you is plugged.
Learn how to sharpen a knife, build a fire/shelter, and skin a rabbit/deer/wolf/forage for leafy foods in your area. Hunting isn't for everyone, but there may come a situation in the future when you are the ONLY PERSON YOU CAN DEPEND ON. If that day comes, you will thank yourself for learning basic survival skills. I highly recommend this book if you're at all interested, it's an exhaustive (but still very compelling) resource on nearly everything outdoorsy, and is the one book I would take with me in an emergency.
Pick up an instrument or learn to draw/write/sing/otherwise spend time by yourself doing something you A: enjoy, and B: are becoming better at. As you progress in whatever art form you choose, you'll feel good about yourself for becoming skilled at it, and you'll have something that you can fall back on to relax with and be alone when shit goes down and you need to think.
LEARN WHEN TO SAY NO. Had too much to drink and your friends are bugging you to drive home? Not a good idea. Have a long-term girlfriend and some cute girl is bugging you to "hang out"? Tell them to have a nice day. Seriously, having self control and ability to know WHEN to practice restraint is both one of the most attractive qualities to the opposite sex, and very important to life down the road. Learn how to judge a situation and to choose the right options based on that judgement. This one takes time and a lot of mistakes, don't beat yourself up too much as long as you legitimately had good intentions for whatever you chose.
Save your fucken money, brah. Being able to keep that hot plastic embedded in your pocket will do many things for you, including, yunno, being able to pay for more important things you may want down the road (a house, a fast car, a rowboat, that houseboating vacation you've always wanted to take in the Shuswap). Life is all about choices, and being able to delay gratification is a VERY GOOD CHOICE if you know when to do it. But also remember that the biggest savings account in the world won't buy you happiness. If you want to learn to drum and there's a sale on a quality kit on Craigslist, and you can afford it, then you know what? Go, have fun. Cute girl in your college science class wants to take a weekend road trip somewhere? Spend the money on gas and make a memory! Life's too short to be scared. Just be smart about what you do with your money.
Robert Heinlein had this to say: "A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects." Learn everything you have the chance and ability to. Have fun. Make stories, love, and popsicle stick crafts. And don't forget to forgive yourself when you fuck up.