You have to be 17 and have good grades. Most apply at 17 and go to college at the age of eighteen to study full time after they are finished at the highschool level.
Hi Phillipa,
The "traditional" age for students to attend an undergraduate program is 18, however there are students who attend earlier. And most institutions also have students who attend later....
If you wish to apply to a college, the first step is to go to the college's website and look at any conditions of entry, fees, entrance examinations and other requirements for joining that college....