So here's my thoughts on the idea of a five year old in diapers...
First, as an ABDL, I know we all have thoughts about what parents should/shouldn't do to encourage their children out of diapers. Having raised kids myself, I can tell you most children get over the poop thing pretty quick. I mean, a two year old might poop himself, but a three year old knows the smell and sensations are not something that they enjoy. Consequently, most of them will train out of pooping their diapers with a fair amount of ease. As for pee, that's a whole other thing. As many have said, some take it way past the normal age of 2-3, possibly because of the ease of using a pullup for nighttime bedwetting, and daytime accidents, as well. Most kids, though, have normal feelings of shame, and/or humiliation, at having to wear diapers, like a baby, beyond the "normal" age of training. They have friends that react to finding their friend still using diapers, and, of course, schools have regulations about kids showing up for kindergarten in diapers. It places way too much stress on the system of teachers/admins to deal with kids still in diapers, so they're oftentimes not allowed to start schooling, until trained. This, of course, adds further humiliation to a kid, to be set back further than their friends, siblings, and all of these things help to contribute to help a kid learn that it's just easier to use the toilet, and flush, and all their troubles go down the drain. It's just easier.
So, why the kid in diapers, at age five, as the OP mentioned? Who really knows. Developmentally disabled? Maybe. Mother not good at dealing with training? Possibly. I mean, whose business is it really? The kid might just find security in them, much like many of us do, and maybe the mother just doesn't want to deal with the perception that she's a bad mother for not dressing her kid in something he/she needs, PERHAPS because they had a few (on purpose???) accidents, and pullups are just plain easier than going through all the consternation of arguing over controlling them out of the protection. I don't claim to know, but my thoughts are that most kids that develop, as their peers do, will be out of diapers long before the age of school. Exceptions exist, but it's not the rule. As some have said, parents are just a whole different thing than they were several decades ago, and that doesn't make them bad people, just a bit lazier than previous generations of parents, I'm guessing.
Truth is, I won't be a hypocrite about it all. If I had to do it over, I might have tried a couple on purpose accidents, to see if my parents would have come up with the diapered solution. In my day, it would have been really rare for them to come up with that solution, compared to the wetting alarms, and plastic sheets, etc., even though Sears had bedwetter pants for sale by catalog. Only, I would have come up with the idea that I should be back in bedwetter pants / diapers. Of course, I always felt that way, from the age of two, but, as I said, my own personal humiliation and shame at the thought of being somewhat behind my peers would have totally freaked me, and, instead, I played out all my diaper desires in my brain.