There's not a steady age when male endlers will develop their coloration and patterns. Does the father have a fork tail? It also can depend on the mother's genes wether the male offspring gets the same coloration and pattern of the father. The genetic background of both parents are in play for the phenotypical outcome of the offspring. Even if a certain trait of the father is Y-linked, which means in general that all his male offspring will show the same trait(s) of the father, can be put down by a dominant gene or genes of the mother. As long as you don't know the genetic background of both breeding parents, it will be a guess what the outcome will be in the offspring.