Once tortoises are about 6 to 8 years old and are longer than 12 inches, they will be sexually mature and capable of reproduction. Size is more important than age, but size will depend on where the tortoise originated.enerally, about 150 square feet is suitable for one adult tortoise and an additional 75 square feet of area for each additional tortoise. If the grass in their yard is holding up to their grazing, you have sufficient room. If not, you should enlarge the enclosure or reduce the number of tortoises.