I haven't kept very good track of when they show sexes and I've usually given them away before they get that big. There is usually a mix of ages growing out as well. I'd say probably around 3 inches you would have a good idea. Also, I keep mine in a growout tank with lots of water changes and they grow much faster than the ones I keep in tanks with other types of fish.
One thing I noticed when I was growing a couple of males up to breeding size in a 20 gallon, the one in the picture above had nice bristles and the other had smaller. When I moved the one with the smaller bristles into a tank of his own, those bristles grew larger. I don't know if it was a coincidence that it happened after I moved him, or if getting away from a more dominant male led to the growth.