Haddaj is correct. As long as they are in good conditions (weekly waterchanges and not being overfed or underfed) they should become sexually mature in 4-6 weeks. Of course the majority of them will be closer to the 6 weeks whereas only a few, normally are early bloomers.
Sidenote: Dino was telling me about one of the Atlanta club members doing a study with video camera on the breeding patterns of endlers, which behave much like guppies. His study found that more than 90%-95% of the insemination was being done by the same male, and that he was fending the other males off of the females. Kinda like a lion's pride, I guess.
