How do i know if my dog is old?

i found a dog in the streets and she is always sleeping and the only reason she gets up is because she gets hungry also she jumps up if she hears the door open her teeth look clean but she is missing a lot in the back and she has one tooth that is rotten how do i know if she is old or not she does not wiggle her tail and when i call her she does nothing just looks at me and walks away