I have had big, grown roosters cover very young pullets. I immediately separate roosters from those pullets if they are younger than 6 months because I had a 16 week old pullet that got her wing broke this way once. A rooster that is aggressive covering hens can hurt or injure a young pullet (or for that matter, a full grown hen). When the pullet is just past the "chirping" stage and before she has laid her first egg, IMHO, a wise rooster does not cover such a pullet & shouldn't.