My mare is now 20, we showed all-around events for years. We are now showing WP! Never too old to change their job. Well guess I should say as long as they are willing! Lol (My mare won't jump now or never had a interest in it even when she was younger!)
We mainly showed English events for AQHA, but she refuses to move out and rather go slow and rock back. It does depend on the horse though as I agree WP is super hard on the joints and hocks. You will just have to see what your horse's opinion is, trainer or vet for a professional opinion.