That's a very good question. Breeders who keep their roosters separate feed them flock raiser or some other formula, but not layer. People have, however, been keeping their roosters in with their hens and feeding them layer feed all their life for a long time. I've never seen anything...
Layer feed should not be started before 15 to 16 weeks, in order to prevent health problems. Here is a link which discusses this extensively. Scroll down near the bottom for this recommendation:
http://www.hyline.com/aspx/redbook/redbook.aspx?s=5&p=36