You are only 20. You should not worry about your age at all.
Art school is ¼ about the skills and tricks you learn there, ¼ about being forced to put something in front of others who won't be wooed by fancy eye candy, ¼ about creating and finding a network of people and ¼ being exposed to other styles of art. Some of those things are influenced by the size and location of the art school, some of those are influenced by how much time you can spend with your collegues and at exhibitions or similar. Make sure to go to a school, where people are better than you. Going there just to learn the handywork is a waste of time. You should go there to learn the skills AND to create a network that can help you get jobs after. Feel free to take on work during your studies.
Also: The things that matter when doing great art are mostly about the brain. And the ability to see the right way takes time to develope. Make sure to do something daily. Maybe a daily comic strip, a daily drawing or whatever. Put it online so you are forced to never skip a day.