It is not possible to tell the age of a tortoise by looking at it. There is a common belief that counting the rings that appear on the shell of a tortoise yields the number of years the turtle has been alive, but the formation of rings on a tortoise shell is not regular.
The rings on a tortoise shell, also known as scutes, develop during times of growth. However, not all tortoises have just one growth period per year. Also, the rate of growth that takes place during the formation of each ring on the shell can differ as well. As time goes by, this can make the rings more difficult to read.
When in captivity, tortoises can outlive many humans, although the maximum lifespan varies somewhat by species. The leopard tortoise, for example, lives as long as 75 years. While mating tends to take place at the same time of year (spring in the Northern Hemisphere and autumn in the Southern Hemisphere), breeding varies depending on the specific part of the world where the tortoises live, which may contribute to the variance in the growth periods for different tortoises, making the growth rings an unreliable tool for measuring age in years.