In my opinion, the older they get the better. Now that doesn't mean that there won't be a certain amount of whining about being tired and can't walk anymore. My litmus test was to watch how they reacted when something they really wanted to do came up. They would whine and complain about how exhausted they were for hours and when we finally would leave the degree of speed and energy they were able to generate to get into the hotel pool was similar to a tornado. They have a way of being selectively "exhausted".
That's not to say that if they aren't fairly active generally, that the new routine will cause a little fatigue, but, unless there is a serious medical issue, it probably won't kill them. Just imagine how you would feel walking around on those young flexible, pain free legs. Just as a side note, that I think helps is to remember to slow down your pace. They have to take two steps for your every one, so whatever pace you maintain becomes twice as much for them. The legs are strong, but, they are also very short. No long strides are possible. Take your time and there should not be any problem and the real bonus is how fast they fall asleep in the evening.