It all depends on the horse. My old trainer had a 26 yr old gelding that her daughter was still using to race. He was in escellent condition and he continued till he was 31 before they retired him...just because he had been working for 28 yrs and they figured he deserved it.
If they are healthy I don't believe they are EVER to old to barrel. My guy is 26 and still winning my friends daughter buckles....he loves what he does :)
And most performance horses that have been performing their entire lives sorta go into a depressed state when they can't go and do what they love anymore.
There is a 30 YO horse and an 80 YO lady that still compete at local playdays. In every event! If the horse is sound and can handle the physical demands of doing the events LET them. Shalom
Its like saying "when should I retired from my job", if the horse loves what he is doing don't make him quit just because he's "old!" If he doesn't have any health problems that are affecting him and he's not in pain while doing it, there's no need to have an early retirement!