I think the problem with the age is the number of concerts they play during a tour.
Maybe in the future they're gonna play only 40/50 dates per tour with a couple of days off between a concert and the next one and one month between a leg and the other.
I'm afraid he could never reach the quality of the These days tour again but if he will be able to dose his voice I think he can go on for 15 years singin very well