Imo it's not too young to sex, and removing the fish from the water and looking at its vent is not the only way to sex it. The pic is not very clear but my guess is possible female, if you had a clearer pic I could be a little more definitive.
If it looks like an arrowhead pointing backwards towards the fish's tail its male, a female's ovipositor looks rather like the trigger of a gun and points straight down. I would venture to say you haven't gotten much response because of pic clarity. A really good closeup of the vent area is required for a positive id.