Guppies can breed after about 1 month, although if an older male is in the tank with fry, it is thought that the older male can fertilise females even younger than 1 month. This should be avoided if you can. Breeding too young is thought to undesirably stunt the growth of the female, and so reduce the size of her fry. I believe it can also lead to undeveloped fry which can have physical differences from normal guppies (like having premature babies).