I've known horses to be euthanized at 5 years old for arthritic changes.
A horse at 14 years old in consistent work with no preventative treatment or care is almost sure to be arthritic. It's a couple hundred dollars for a lameness exam from a vet, and then you will have a clear diagnosis and can work from that. Any chiropractor worth his salt will not adjust a horse without seeing x rays of the area first, nor would I ever use one who didn't follow this protocol.
Your vet is your best resource for a sound, healthy horse.
They say money doesn't buy happiness -- well happiness doesn't buy horses!