Honestly, I don't think there is a single answer. And even if there was, the question is wrong too; if you genuinely think there is a cut off, it should be by age, not size.
Come on, you've all thought it at one time or another "That child is too old for a stroller." (probably before you had kids) But so what. There is an entire website devoted to showing examples of this (this is by someone who has no children) Walk
I know when I first thought of having kids, I wanted to be strollerless. I wanted to wear/carry my child. Having twins put a bit of a damper on it. Yeah, I was able to wear one of them, and sometimes even two, but a stroller was a necessity. Maybe if I lived in the burbs and used a car to go everywhere it might be different. But I live in the city, near downtown, no car and (almost) everything is in walking distance.
I went through a lot of strollers in the search for the one that worked best for us. And there was never just one at a time and most served a purpose for a year or two and were replaced as they grew and our days changed. (I bought and sold second hand mostly). Right now, we have a side-by-side double, two single umbrella strollers and a wagon. I personally have used the double only once or twice this year. I'd sell it, but my caregiver uses it anytime it's raining (this is Vancouver, it rains).
It used to be the two umbrella strollers were only used with another adult, but now Corwyn loves to push his. We can often go for an entire day with Nathan either riding or pushing his stroller and Corwyn pushing his the whole time. Most of the time. Sometimes though, we can be somewhere and Corwyn decides he can't walk anymore. It doesn't happen often, but after getting stranded and having a friend have to come help, I've started putting the stroller connectors in the bag. I'd like to stop using them all together but they can only walk so far, if the destination also involves playing then trying to walk home, I can't respond to carry requests with two of them.Oh and for those who are thinking to get Corwyn to push Nathan. No. On rare occasions he will, but mostly he just wants to push an empty stroller (he'll push the double or pull the wagon too, but only if it's empty - doing anything necessary to get Nathan out of it).
The boys are 3, barely 3, but 3. Even for those who think kids should be out of strollers before school age think 3 is okay. Nathan is big. In an umbrella stroller he looks big. He looks too old. And now it's summer. In the colder weather in a bulky jacket, it will look worse. I see the looks; I hear the comments said in stage whispers. Get over it people. Nathan plays hard and rests hard. We walk lots, take the bikes some and use the strollers too.
I miss the always in a stroller age sometimes. Getting places was so much faster. Shopping was a million times easier. But I love the freedom of going out without one. I'll be happy when we're totally done with them, and a bit sad at the passage of time to.
Those are my kids. My kids are healthy and luckily developmentally normal. But you can't tell by looking at a child if there are issues that would require a stroller until an older age. Wheelchairs aren't cheap. Or sometimes a stroller sometimes because they can get easily tired because of certain medical conditions. Or they don't have a car and 10km is too far for a younger child to walk in a day.
Or whatever. Maybe the child is lazy. Maybe the parent needs more control. So what. Bottom line, it's none of your business. Is it hurting you? Do you lose sleep over it? Are there no other problems in the world and in your life that this is the most important cause you have? I don't think anyone will be pushing their developmentally normal child to high school in a stroller.
Sorry for the small rant. I was amused one day when Corwyn was sitting and pushing himself along in the doll stroller. And it crossed my mind that someone could post a picture of this on the website saying he was too big.