Well physically they start producing sperm plugs and showing courting behavior at around 5-6 months ( this age may vary slightly from Cham to Cham). But I would say they are able to successfully breed at 6 months , most breeders give them access to a receptive female to breed at around 8 months.
In the wild I'm pretty sure there are 5-6 month old Chams breeding with females they chance upon.