Maggie: You're right, you can't count the growth rings on a tortoise for age like you can on a tree. Tortoises may have different growth spurts in a year, not just one. If he's more or less food deprived during a year, then all of a sudden provided with lots of food, then no food, then lots of food, he may have two or three growth rings that year. You CAN however, use the growth lines as a sort of guide to give you an ESTIMATE of the tortoise's age.