How old do you HAVE to be to start college?

I was just wondering because I want to start college early. I'm already doing 11th grade work and I'm in fifth grade. I might start home schooling soon, so I would get even more ahead. I already know what degree I want to get, a major in zoology, a minor in marine biology,and a PHD in herpetology. How old do you have to be to start college? I need to know ASAP