It all depends on the species of tortoise. Many captive bred tortoises breed much earlier than wild tortoises because they are healthy and growing faster than wild ones. Did you have a specific species in mind?
I believe size is more a factor than age for captive tortoise and turtle....and in the wild alot of tortoise are believed to breed when they are 10-15 years of age....and I have to say I don't believe tortoise in captivity are healthier than their wild counterparts...