I've got two young axolotls, around 4-5 in., when should I worry about them breeding if I've got one of each gender?
Sexual maturity happens at about the 1 year to year and a half mark on average. If your Axolotls are 4 - 5 inches id say they are still way too young to breed. You still got time before you have to worry about separating them. I'd be more worries once they reach around the 7-inch mark.