If they are fed well, in a stable environment, and have minimal stress, they'll spawn eventually. If they have a bunch of fish harassing them they probably won't spawn. That is one reason why people have success keeping them in tanks by themselves with only a clay pot.
I've heard that they will spawn as young as 6 months, but mine took about a year and a half. Good luck to you. There are some great resources here (specifically in the breeding forum).