Telling the age of a turtle can actually be very difficult. Obviously, it's best to know when your turtle was hatched. This is the most reliable answer to your question. You can base some age estimates on the size of the turtle. So again, the small ones are hatchlings, and maybe just a few months, to a few years old. And the ones reaching their full weight for that species are usually of a certain age as well.
In tortoises like this red foot tortoise, you can actually see rings on the scoops of the carapace, or the top shell, here. So some people think that each ring corresponds to a year of life. Unfortunately, this is also unreliable. Sometimes, these guys grow quickly and develop more than one ring a year. I've also seen cases where the turtles were not cared for properly, and their shell actually didn't develop normally at all, and was either stunted or deformed, and you couldn't see the rings.
So in older turtles and tortoises, sometimes you'll actually see that the shell is very worn, with scratches and dents. And that comes can be an indicator that it is an older turtle. But if you're looking for a reliable way to tell the age of your turtle, I'm sorry. There just isn't one.