I've used them until what ever age we found them useful, same with carrying my kids.  If your child is constantly asking, then I'd get a health checkup.  Children by that age, in a fun place, will nearly always prefer the freedom of walking.  I was only ever asked to push or carry if they were tired & we had a long way to go.  While others have said, take breaks, I agree other than it's not always really feasible.  I have had times with kids of all ages & the 5yr old might be done walking just as the baby is getting sleepy & if we stop now, baby is going to cry, if we keep on, baby sleeps & 5 yr old is pushed & life is well. I also can't lie that I like the stroller, not so much for my kids, but for the things I want to take, or a wagon, that works as well, but you need a good wagon then, not the the type that has one hand dragging it behind.  That is not great for your back.  But if someone wants to ride they can, but I will be sad when my youngest outgrows a feasible age to take it along, she is 3 now & never sits in it.  I just take it because it is handy to have a push cart with us.

You also have to look at potential motivations.  My one son hated strollers since being a baby, so he never really rode in one.  Then when I had the next, he was 3 & suddenly he wanted a stroller, so I got one.  He would ride in it when we arrived to any new location for maybe 10 minutes before deciding it wasn't great and then want out.  I *could* have said, "No, that is for babies, you can walk, blah blah blah" but I also knew he was really saying, "Will you go above & beyond for me too?  Do I still matter?  Can I still be little even with a new baby around?"  So it lasted maybe a few months that he would ask, generally he would just ask before we went if he could ride when we got there & then by the time we arrived, he was over that.  It wasn't hard to accommodate & eventually he realized that I still loved him, he still mattered & riding in strollers was really not fun. 

**  I also have a friend whose child has a terminal illness that means he tires easily.  He outwardly looks fine.  He is 5.  She gets so many dirty looks when out with him.  For that reason, I'd never ever suggest any of us think we know an age when something is "too old" when it comes to anyone other than our own.  You can't know what is happening with other people & with her child deteriorating, the last thing she ever needs are dirty looks, yet I've seen people give them.