To be absolutely sure of your turtle's age, you have to be there when it hatches. While it is growing, you can get a rough idea by its size. But for that to work, you have to know what kind of pet you have. Also, think about children who are seven. They are not all the same size. This will be true for turtles and tortoises, too. So even size won't give you a really accurate age. Once the turtle is full-grown, there is no sure physical way to tell the age. You are left with historical investigation.