i'd say when they are starting to grow all there legs and get a bit bigger, i'm no expert but then they become around an inch and a half long or so, but make sure as they grow bigger dont keep all of them in one tank, a 3ft tank is probably okay for two fully grown axies, you can keep them all together when small but as they get bigger, bigger tanks will be needed and more tanks.
hope this helps a little like i said im no expert :happy: