That's one option. The franchise agreements last for five years normally. Once someone moved into their mid-fifties I'd discuss retirement options at every renewal. That way it could be shortened (or slightly lengthened) to hit their desired retirement date. If they wanted to retire on the relevant date, it'd pass over to a new franchisee (though I'd try to phase it over if possible, as starting cold into a full round would be a killer)
However, two of my guys see themselves going on for a long way past normal retirement age, winding down as time passed. After all, as one of them has pointed out, you'd have to be in a bad way not to be able to do a couple of mornings a week while in your seventies and have a cracking income (for a retiree). If they wanted to do that I'd do everything I could to make it work.
Hope that helps.
Vin