20 is a milestone age for most. It's the time when you are no longer a teen and not yet a complete adult. You are most likely to still be in college. However, all that will be in constant flux in the next 3-4 years where you have to decide on your career choices, get a job, start earning (and saving), and being independent in the truest sense of the word.

Let's break down the things one need to know when they are 20 into basic survival traits and the 3 major spheres of life - personal, social and professional (I have taken this partly from my post. You can check it out too if you like although it does have things you need not know before 20, but later on) (Links have been underlined):

A. Survival and self-sufficiency

1. Cooking
   Face it, this should be taught in school. If you can't cook, you are dependent on others - your maid, your spouse, your local restaurants. What if it is late night and you are alone? Where will you be when hunger strikes? (:P). Stop thriving on easy-to-eat food.
   (Get started , Website, Android app)
2. Budgeting and managing your expenses
   

• How to save
• Log and track your expenses
• Get the lowest price for everything
• Get free stuff

1. Basic survival skills
   Imagine you are in an accident or a crash. Your chances of survival will solely depend on how well you can analyze the situation and use everything at your disposal to help you live and get out.
   
   What you should know?
   

• Knowledge of plants and animal - edible, poisonous
• Knowledge of the major ecological spheres - forests, oceans, deserts, snowy mountains.
• How to conserve water
• How to build a shelter
• How to build some basic weapons
• A good signalling mechanism
• How to avoid predators
  (All the above and more)
  

1. Self-defense
   The world can be a nasty place sometime. Know how to protect yourself and your loved ones if ever the need arises. (A good starting point)
   Enroll yourself in a self-defense class. Have some basic training in martial art form. Build up your strength and agility.
2. Learn how to jump-start a car
   
3. How to set up a camp
4. How to make a fire without matches
   
5. Emergency skills
   

• Performing CPR
• Know the warning signs of suicide so that you can help someone
• How to perform the Heimlich Maneuver
  
• Know the warning signs of a heart-attack

B. Professional career

1. Know how to build a good resume (Resume tips 1, Resume tips 2, Resume tips 3)
2. How to apply for jobs and increase your chances of selection (Reading)
3. How to make a good first impression (Reading)
4. How to make good connections at work (Reading)
5. Have a secondary marketable skill
   What if you were an investment banker and suddenly found yourself without a job after the 2007 crisis?
   
   Always have a backup. What else can you do except for trading and valuations? Can you code? Can you design something? Can you write? Can you work as a salesperson?
   
   You should always have a secondary skill you can earn with. You might not be as adept at it, the industry might not be as well paying, but it will still save your skin in times of need.
6. Learn how to manage time and have a healthy work-life balance. (Reading)
7. Learn coding.
   Nothing can be more helpful in you day-to-day professional life than knowing a bit of computer sorcery. Know how to write the simplest of macros to automate mundane tasks, be efficient, remember all your To-dos. You have a pretty powerful machine at your disposal. Make it work. (Get started)

C. Personal life

A person should be able to live alone - with no external contact or assistance. It's essential to take out time for family and friends, but equally critical to take out time for self.

1. How to build stuff from scrap and basic household items (The best resource on the internet)
2. How to stay fit without any equipment (100 no-equipment workouts)
3. How to stay alone, yet entertained (74 things to do when you're bored and alone)

D. Social life

You might be an introvert who spends weekends in his living room, reading or surfing through channels. But contrary to point C above, man is after all a "social animal" and must know how to live among others.

1. Know how to break the ice (Reading)
2. Know how to strike up and hold an interesting conversation. Sometimes with complete strangers. Probably on topics you know nothing about. (Reading)
3. Learn to listen. Listening may seem like a passive ability, but it isn't. There's a difference between hearing and listening and that is the level of engagement you have with the speaker. (Reading)
4. Get people to like you (Reading)
   
5. Learn to make cocktails and be the life of the party (Some easy recipes)
6. Learn to make some easy-to-make snacks for when you are having people over (Some easy recipes)
7. Public speaking. Join Toastmasters.
8. Learn card games. May be poker? (Know your poker hands)
9. Learn sign language (American Sign Language)
10. Know how to remember people's names and faces. (Reading)
11. Also, how to make proper introductions. To avoid awkward situations. (Reading)
12. Or, how to tell a good story (Reading)

E. Misc

1. ONE BIG ASS CHEAT-SHEET FOR LIFE
   
2. A compendium of useful psychological life hacks
   
3. How to write fast, well-constructed papers
   
4. Never miss your lines while delivering a speech
5. Kitchen cheat sheets
   
6. Dining Etiquette 101

Finally, the mandatory quote: