What are the indicators that allow us to estmate someone's age over the phone?

Ok, kids and teens are pretty obvious, but when I take a call, what is it that allows me to guess the age of the caller? It’s not foolproof, but I can usually say something like “She sounds like she’s in her 20’s/40’s/60’s” and when I think about it, how could I possibly tell? Is there something in the tone that give a hint? What would that be? I don’t think it’s behavioral; then I just think “she sounds very professional for her age” or something along those lines. Is there any information out there?