I bought my old dressage horse at 16, pootled around with him for a few years, then moved yards and was told with his history, breeding and experience I could/should have learnt so much more- so we changed form happy hackers to dressage riders.
At 19 I sold him to a friend, not because I didn't want to keep him. My trainer sat down and told me 'Girl, you have talent. If you want to ride as a sport, you need a younger horse with younger legs- Doey is older, and with the intense work he did as a younger horse, its taking its toll in the cold weather and its unfair to keep him going like this.' I was beside myself, spoke to my friend and she said 'I'll buy him!' so she spoke to my trainer, who is her trainer also, and bought him. Was I distraught? Heartbroken, and sometimes I get to ride him, but he's gone to a loving forever home. If I could have afforded two, I would have done.
But I would NEVER have sold him to someone who would ride him for a couple of years and sell him on. He's fit, he gets a bit stiff in winter, but the only time a vet has been out to see him is for injections. Had he been a mare, I would have bred from him.
Older horses may require a longer warm up, some possible meds to keep them ticking over, but a friend's horse had arthritis at 6.. it strikes at ANY time. Don't buy to sell on, but to keep at that age.