The last trip any of my children rode in a stroller was age 5. But you will see kids but in strollers that appear to be old enough for Jr high.
I would bet that some who we see in strollers might be 9 or 10 or even older- but that they have physical/mental limitations that we might not outwardly see.
Now, IMHO- if your child is a healthy child without these kinds of limitations- perhaps once you kid hits 8...I'd say they need to grow up and stop being lazy. I see more and more kids these days just being lazy. LOL!
People have suggested on the boards to prep your family- especially your little ones by walking at home for 30 minutes a few times a week. Build up stamina!