Being able to draw is definitely a big plus, but not a huge requirement. My own drawing skills aren't all that great, but passable.

20 isn't too old. I only really got going around 20 myself. Took me a good 2 years to really start to get it right.

School isn't required to get a job in this industry, but having a great portfolio IS. If you feel you need to go to school to learn what you need to make a great portfolio, do it. if you think you can do it on your own, more power to you!