Yup. Age-related hearing loss can start to show up at 15, sometimes younger in horses that are exposed to loud sounds a lot (some show horses and horses used in battle reenactment are at risk - think about this for your writing. Artillery horses are likely to end up deaf!).
They generally compensate pretty well for it, though, and horses are less reliant on high frequencies than some animals, so may not even notice the first stages of hearing loss.