I would agree with the walking/non walking being due to psychological/personality aspects, at least in some cases.
DS (3.3) is still very firmly in the buggy, although can have a jolly good jog around the park and a good long dancing session at home.
I think it's due to a "non-bouncy" careful nature and laziness/lack of will to invest energy on things that he can't see the point/fun in. For example, he doesn't see the fun in walking on the street, whereas if he has a buddy with him, he will walk at least some of the way, as it's more fun to skip along with a friend than walk boringly next to mummy (mummy can't always motivate or find fun as well as other little ones, although she occasionally tries).
Also, DS nearly always refuses to go out in the first place (again due to his personality not being very outgoing or adventurous), so if I didn't put him in the buggy, I would basically have to carry him (and I have a bad back) or drag him along the pavement (child abuse). He very rarely responds to any kinds of bribing or punishments, and gets hysterical if I pretend to leave him sobbing on the pavement, so he will stay in the buggy until he feels too ashamed to do it (or doesn't fit in the pushchair- but he is not very big for his age and weighs only 13 kg) or can be bribed out of it. It's fine with me, as he gets enough exercise anyway (doesn't have weight problems, either), and we don't have a car. I notice some people doing this
at us already, but I'm sure they didn't have a child like DS to deal with, so screw them!
My aunt stayed in her buggy until she was 5. My grandparents were embarrassed (as she was very tall for her age, as well - is now 6 ft), and feared there could be something wrong with her. Turns out that she was just a bit lazy (with other things too, like talking, eating etc. - this sounds very much like my DS!), and actually just very intelligent and could not be bothered with the "mundane"
. So there might still be hope the non-walking children.. 