I've been asked that question many times as well. I didn't start college until I was 21 (I'm 24 now). I delayed college because I didn't feel I was ready. I was too depressed all the time and couldn't focus. Actually, I'm still the same but I drag myself to classes anyways.

No one can make up your mind for you. But you'll know when you are ready to start college. It's something you don't want to start unless you are ready for it. You don't want to be forced/feel pressured to go otherwise you might not do well. And if your first semester at college is horrible for you to go through, you won't want to continue and you may never go back. So I suggest you wait until you are ready. If you are clinically depressed, try to seek out help and discuss the problems you are having and maybe medication and/or therapy can help you become more mentally/emotionally happy and then you might want to start college.