Pretty much what the OP says. How old does a tortoise/turtle/terrapin have to be before before it can have children?
Bob also asks the question “Where do tortoises keep their testicles”?
We have an … interesting marriage. 
They don’t – it’s goats that have kids. 
Seriously, the giant tortoises (from Aldabra & Galapagos Islands, for example) seem to start breeding once they reach about 1/2 of their full size. Since this would depend on many things in their environment, such as the availability of food, etc., this would tend to vary a fair amount, rather than being a specific age.
Reptiles keep their gonads inside their bodies - the scrotum is a mammalian thing.
What t-bonham said.