Chris thank you for your question. Some people suggest that it is the number of scutes (the bony rings that the tortoise has on its shell. However look at this baby tortoise breaking out of it’s shell.
Here he/she is and already there is more than one scute. Maybe we need to take a hint from a tree. How do you find the age of a tree. When you cut a tree down you can see inside of the bark a series of rings. (You can obviously do this without chopping the tree down). Look at the image below.
The scutes of a tortoise also have rings. The image below is an ‘old’ tortoise.
This image is of a very young tortoise
Maybe these will give you a clue about the tortoises age. It is thought that this could be a rough age indicator.
Alternatively you could get a size chart, however, like the rings it is difficult to accurately determine the age of a tortoise. Maybe the only true way is to know when it was born.