I found an 6.5in yellow-bellied slider, what basic care I should know and about how old it is?

Best bet would be to contact a local vet and ask them if it needs any sort of special care, diet needs, etc. I know the 1 turtle I had to care for while house sitting just ate some pellets. They had it in a fish tank with an island that had a ledge far enough below the water line for it to climb onto.