Ya they mature at 12" but most rays get discks of 3' eventually. They usally slow down around 24". Even retics (falesly called teacups) have small discks but they have LONG LONG tails and they grow slow so it is a long time till you can breed retics. Motoros mature in 2-3years usally and are the hardiest and easist to breed.
yes i know.... but the chances of him getting a scobina are slim to none, and i would rather give him information the set him up to succeed with a bigger tank size