The earlier you can begin your promotion to college the better. Many coaches like to see the development of athletes over a period of time and see that they are committed to the idea of playing in college. A lot of coaches also like to begin recruiting in advance and want to begin chatting to players 12, 18 or even 24 months prior to an athlete entering college.
Beginning promotion earlier also increases the amount of exposure you can get to coaches and allows a greater range of colleges to receive your information and analyse you as a player.
There could also be more specific academic entry requirements, so if you are being promoted earlier then academic classes and eligibility can be assessed and changed if required.
The more promotion you can give yourself to coaches, the more a coach can get to know you as an athlete, academic and person and thus giving them more confidence in your abilities within their squad.