Unless you know the day it hatched, you cannot ever tell the age of a tortoise.
They all grow at different rates depending on whether they are wild caught or captive bred and depending on the diet and care they have received since they were born. Any age given is a rough guess based on size and will be accurate to ten years or so either side at best.
Joe's Mum gave you spot on info! If you post a picture and it's a young tortoise, we can give you a good guess, but if it's an adult, there's no way to even guess.