I slightly disagree with Franks post on piranha maturity, however I may be wrong. I believe most Pygocentrus species come into maturity sooner than Serrasalmus species because Pygos life span is much shorter than that of Serrasalmus. Proof of this matter would be the average growth rate of each type. I had a piranha (which died on christmas morning unfortunately) that was a female Nattereri. She was about 8.5" and was about 1.5 years old at her death. She had began to carry eggs at just over a year old. She never actually bred because my male took alot longer to mature than she did. One time they were preparing to spawn but I had to move them to another house so that ruined that whoel process. But my point is that Nattereri can come into maturity around one year of age. This may not be the case with all Natts or all pygos but it could vary well be the case. I know Machete of Pfish had Natts breed at just a little over one year in age. The reason I know is that when my female began getting eggs I couldnt believe it, but he confirmed that indeed it is possible. It is highly possible that some Serrasalmus come into maturity at 2 years but my opinion is that it takes most of them about 4 years or so, afterall some Serrasalmus are only around 4"-5" at 4yrs of age, hardly big enough to carry many eggs if any.