The way6 to tell a slider turtles age is not an exact science unless you have hatched the turtle yourself you have gotten the turtle form a breeder the color changes over the time of growth is how we estimate the age of the sliders. The shell(carapace) turns colors as well as the barrs change color too. I hope this information will be of help.Joan This is an except from :
Central pets on how to tell the age or a Slider turtle:
Bright bands on the sides of the Red Eared Slider's head are where they get their name. These bands are not always red, often varying from orange to an almost brick red color; and sometimes they do not appear at all. These bands may also be broken into a few spots and can fade with age. They sometimes also have a spot the same color as their bands on the top of their head. It is not rare to find them now as albino or pastel varieties. When Red Eared Sliders are born, both their carapace and their bodies are a vibrant green. In young adults the shell will often go through a yellow stage, and eventually become olive in color. The carapace at this time is also covered with dark patches, streaks, and lines, as well as patterning in yellow, white, and red upon occasion. The carapace changes colors with age, often turning brown, gray, or even black; and after a time, you have to look closely to see the bands on the sides of their heads, which fade in with these colors. A Red Eared Slider can reach an adult length of anywhere between 5 and 12 inches. Females tend to be larger than males. Males have longer tails than the females and have long front claw nails, which they use in their breeding dance.
Telling the age of a slider turtle is not an exact science. The size of the turtle will give you a vague example of age since the females and male can only breed at approximately 5 inches which is somewhere between 5 and 6 years of age. The feet and tail will help detering the sex of a slider, but the only way to get an idea about the age is to watch the color changes in the body and shell and the the way the bars and lines break up with growth of a turtle.
I will opt out since you did not find my answer helpful and allow someone else to give you an answer that may be easier to understand. Respectfully, Joan