To me, it really depends on the horse. I've seen some 18 year olds that I look at and say "they're old," then I go look at my 25 year old and say "she's getting there." Some horses just age faster/worse than others and that has a lot to do with how old is old, in my opinion.
Basically, once they stop being able to work very much/hard and they can't keep muscle tone, that to me means old. However, if they still have some fire in their belly and they look youngish, like my mare, I don't think of that as old no matter how old they really are.
Hopefully that makes sense. Haha
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.