It all depends on the horse, but thoroughbreds do tend to get arthritis and other problems as they get older, off the tracks especially
Whatever you thinks right
I have a 19 y/o who was out of work for 4 years before I bought him as an 18yr old. He is now eventing comfortably at 80-95cm and is still going up in quality. At the same time though my mums horse went downhill from 20-22 until he had to be put down. Depends on he horse
Hey it really depends on what the 18 y/o has done if he has not done a lot I would get some younger if you want to go far.