One of the first things you should know when choosing a turtle is what age he or she is. There are different ways to tell the age of a turtle. One of the easiest methods to estimate the age of your red-eared slider turtle is by its size and color. For turtles under 6 months, their shell is less than 4 inches and their coloring is bright. Adult slider turtles have a much larger shell, and their color is a little paler.