As far as the Ender’s producing more females, I can only guess. Are you sure they are mature enough to be certain of the gender, are they gravid? Gender is genetically determined. I think that if there are excellent conditions in your aquarium, possibly pheromones are being released that cause you to get more females to build the population of a colony in order to dominate. I’ve noticed when only 2 fry are produced often they are 1 male and 1 female. The release of pheromones may have something to do with survival of a species as well as affect the fertility of male sperm. I’ve seen this with Livebearers as well as mouth brooding cichlids. I’ve read that a higher pH will yield more females. I’ve read that higher temps can reduce the survival rate of male sperm. There is a lot of anecdotal information out there. Just go with your observations. If you’d like to change the gender ratio then you’ll need to do something different with hardness, pH, temperature, lighting intensity as well as duration.
It really depends more on the your perspective which is usually based visually on size rather than age. Approximately any swordtails less than ¾ “ would be perceived as fry and any larger than that would be considered Juvies, once they are mature enough to begin or start dropping fry Young Adult Size and Larger Fish would be considered Breeder Show Stock.