red ear slider turtles?

Question

how can u tell how old they r

Answer 1

It is really hard to tell unless you have had them from day one.

But here is a guide
Newly hatched are the size of the egg (Not very big at all)
Hatchlings - up to 3-4"
Sub-Adults- 4-8"
Adult - 8" or larger

Here is a site that is very useful and has more info
http://www.turtlecare.net/age.htm

Answer 2

http://www.austinsturtlepage.com/Care/caresheet-red_ear_slider.htm

It can be difficult to tell--you can't tell by size because a turtle that is not properly cared for will be sick and not grow right.