A question about breeding brahmas

My brahmas will be 21 weeks old this Wednesday. A couple of my light girls started laying at 19 weeks old, but it's still sporadic; like an egg every other day. The lights are huge for the most part. My buffs and my darks are smaller, don't look as mature; with the exception of one dark pullet and one buff.
My question is, since the brahmas mature so slowly, would it be okay to let one hatch out chicks in the spring or should I wait until their second year? This is assuming of course that I have a hen willing to cooperate with the broodiness. At this point they all like to play at nesting, but the girls that are actually laying just jump in, lay the egg, jump out.
Thanks!