"When did you start college?"
Translation:¿Cuándo empezaste la universidad?
"Colegio" means grade school, not college. "College," (at least in the American sense) translates as "universidad." You do not use the preposition "a" after comenzar, unless you are adding another verb: "¿Cuándo comenzó usted a ir a la universidad?" or "¿Cuándo comenzó usted la universidad?" Use of "usted" is correct, but if you are talking to a college student, you are far more likely to use "tú."
comenzar and empezar are close enough, so sure why not. However I am not use why you have used "a el" in your sentence?