With DD, our stroller broke when she was 2 and we never replaced it, and never missed it. Spent a week at WDW (in the hot summer!) when she was 4 and she never once complained of being tired.
We did however, use a stroller for DS at MK last year (he was 4) because we were in the park for 18 hours and I knew he wouldn't make it that long. The stroller was given away immediately after that trip. (Ironically, on the trip when he was 2, he spent a total of about 3 hours in the stroller during the whole week, and walked the rest of the time)
Personally, I wouldn't allow my kids to ride in a stroller any past 5. Kids are supposed to have more energy than adults. If they can't keep up, something is wrong. Make sure they have good shoes, are eating healthy meals, and staying hydrated. And of course are getting regular exercise at home. If they are otherwise in good health, and all of those things are happening, there should be no need for a stroller.
I know this can be a heated debate, but that's my $0.02 anyway
And keep in mind that if older kids are in a stroller, they WILL get funny looks, and maybe even comments. Obviously that's not cool, and I would never do that or let my kids do that, but unfortunately it will happen. And it's not something I would want to subject my kids to, if I could avoid it.