Shoot, in this day and age, 17 is still young! If he is sound now and doesn't have any health concerns, it is entirely possible that he could continue to be a very productive and competitive mount for 10 or more years.
Heck, there is a member here, Wallaby, who has a 27 year old mare that is just as spunky as any young horse and she rides her quite a bit.
Actually, riding more often and keeping them moving and fit is what helps to keep an older horse feeling younger. It can help to stave off arthritis and other health concerns that come with lack of activity. Horses are much like people, if they stay active and social, they stay healthy, but when they start just staying at home and laying on the couch (or standing around in the pasture) then their health starts to decline quickly.