I doubt they'll "stop growing" but it could be dangerous to the fish to breed too young. Think about it, a female guppy can give birth to more than 100 fry at one time and they still struggle when fully grown; subjecting a juvenile to the stress of being chased by males, then giving birth is, in my opinion, quite cruel. I always seperated young male and female guppies when I used to breed them for this reason.