From the information I could find, there is no definitive way of determining the age of an adult yellow-bellied slider, unless you've owned it from a baby. Counting rings on the shell is not significant in determining years of age.
For young females the yellow mark behind the eye is more striking, as is the underside of the belly. These feature tend to fade with age. The males tend to grow darker.
So without asking the previous owners if they know how old she was when they took her, it's hard to guess. It is understandable that you would want to know. They can live for many years, but it would be good to know where on her lifeline she is.
They are brown or olive green, usually with a prominent patch of yellow on the side of the head. The lower jaw is rounded. The shells of yellow-bellied slider turtles average in size from 5 to 8 inches; the record is about one foot. The yellow blotch behind the eye is the most conspicuous marking and is most prominent in juveniles and females. The yellow underside of the turtle's shell sometimes is marked with round dusky smudges; these markings may be reduced in older turtles. Also, adult males may become very dark.
Yellow-Bellied Slider Turtle Fact Sheet Factsheet produced by the Outreach Program of the Savannah River Ecology Laboratory, The University of Georgia
Yellow-bellied sliders are omnivorous, however, juveniles tend to be more carnivorous than adults.
Yellow-Bellied Slider Species Profile ReptileChannel.com