Well age/size, along with tank conditions, certain lighting conditions, cooler water (they do come from deep in the lake). It really just depends on the fish and the conditions. I have talked to quite a few breeders who have told me how they breed their fronts and everyone is different and everyone has their "tricks". The variant may make a difference as well. As for age/size again I think this also depends. 2 years is generally what most say but I would take that as the earliest they may start and not when they will start, it could be/probably will be much later. If you know the variant I would do a lot of research on that variant and try to talk to breeders (some may not tell you everything though). There is just a lot of considerations to take into account for breeding them.