They will start to breed around 2-3 years. If you have a dug pond just keep adding catfish and water until you get fry. They usually spawn between april and june and only do it one time a year. If the conditions arent there (temp, volume, and food) they will not spawn. I usually notice fry around the middle to end of June up here in NNY.
I think that in an aquarium, if you keep the conditions of your tank cooler then raise to breeding temps it may spark them to breed. You will also need to have enough substrate for the male to dig a deep enough nest that will look appealing to the ladies.