I don't go by age, I go by size, shape, and activity. I wean them when they start foraging on their own, and their shape is that of a small adult, not a plump, round hopper. I will usually introduce a male that is around the same age at this time, and let them mature together. It usually takes at least a month before they will start mating, regardless of the bodies maturity, and in this time, it helps them to establish a bond and a family unit. It also assures the male that all offspring are his own, and makes him a good father. Or you could use a larger male, who will become "protector" to the young females, and will help them mature faster, but still accomplishes the desired "family unit" a strong colony needs.