It has already been said, but I will say it again. A lot of the "bigger" kids in strollers may have a disability or difficulty that is not visible to someone else. That is the case with my DS5/autistic. I'd much rather push that 50 lbs than have that 50 lbs pulling and tugging me all day, or spontaneously falling to the ground for no reason and taking me down with him, or dealing with that 50 lb meltdown...
There is no need for this to be a heated subject as someone put it. To each his own. If you want to push your 12 year old in a stroller, it isn't hurting me. If your 3 year old can go all day without a stroller, then that's fantastic too. Everyone's situations and abilities are different. BTW, my DS gets plenty of time out of the stroller with plenty of walking time so he is usually falling asleep before we leave the park along with my DS6 who walked the whole day. Just trying to offer my perspective.