Is there a way to determine a turtle's age by simply looking at it?

I was skateboarding around the neighborhood and found a HUGE turtle just sitting in the middle of the road. The stupid thing was just sitting there... So I manage to get him on the edge of my board and bring him to my house. My mom is going crazy and is bombarding me with questions, one of which being, "How can you tell how old he is?" She wanted me to look it up, but Yahoo! Answers should suffice her curiosity. So, is there anyway I can figure the little fellow's age by just looking at it? Or perhaps a good estimate?