How old is too old to break to harness?

My ex stallion (gelded early teens) took to driving like a duck to water. Poor guy has had many stages in his life…show horse, halter, pleasure (west & eng), stud…then gelded and used as an all round ranch horse…all of which he did well, but in his late teens he showed me what he wanted to be when we both started taking driving lessons. He always had an attitude. You didn’t ‘tell’ this horse, you ‘asked’ . Although an APHA, his dam was an appendix mare who was Native Dancer bred and it seemed his reputation may be true in our case. My instructor/trainer was experienced, showed in carriage shows and did exhibitions of trick training but my gelding decided he didn’t like him and they would get into conflicts. " You will, no I won’t " and if you make me I’ll throw a fit…he was already hitched and going fine so I quit lessons.
Luckily we went to driving clinics and he loved being a show off, often chosen to be the demo horse for harness fitting, long lining and demonstrating the different speeds of trot. Great fun for a couple of summers. Did a couple more years of trail driving and now retired at 25 running around on hilly pasture and woods with his hackney pony buddy.