As TQ says, if you got 17K on BOTH front and rear, consider replacing BOTH...3 or 4 years on the same tires may have hidden defects like "age cracking on the sidewalls and such"...also consider looking at the DATE CODE to be sure it is not a couple years older than you think. Latent defects are just as much a potential problem as well as how they were used. Rubber compounds do deteriorate with UV, heat and ozone exposure, which may not be visible from the outside. Also, the round profile may be squared due to lots of straight up riding, so transition may make bike wander and wallow when pushed...and a front tire flat if spoked wheel can be sudden and dangerous with the inner tube quality hidden from view.