I don't think that the second a horse turns a certain age that they are "old".
I don't consider a seventeen year old horse to be "old" necessarily, but perhaps a bit aged, if that makes sense.
Horses can live well into their twenties and even their thirties. So long as he is still happy and seems healthy there is no reason to worry. :)
My first horse was twenty when I first got him, and is still living in a horse retirement home six years later.
Tango - 6 yr Sorrel Quarter Horse Gelding
Corn - 8 yr Gray Quarter Horse Gelding