This isn't meant to sound creepy, weird, or sexual at all, simply unusual.
My job involves me visiting clients houses and taking measurements. Yesterday I had one such appointment, at 2pm. It's summer school holidays in Australia, and hot.
The house I visited had a young-ish mother, and four kids there. The kids had all been watching a movie in the bedroom, but started following me around and asking me questions as I went about my job.
There was a boy who was obviously the biggest and eldest, and it turns out he had just turned 13. A middle girl, and 2 younger boys. The 2 youngest boys were extremely similar in height and appearance, although one was a bit larger. The 13 year old wore sweatpants but no shirt, the girl had a dress on, but the 2 youngest were wearing only diapers, and nothing else.
As the kids were following me, the mother took them into the lounge room and gave them each some cake, which the biggest boy explained was his birthday cake, and he'd just turned 13. This was how the subject of their ages came up.
When the mother and I left the room, I asked her if the 2 youngest boys were twins, as they looked so similar, although not identical.
That's when she dropped the bombshell, telling me the smaller boy was 3, but his brother was 5.
I was actually gobsmacked to see a 5 year old happily spending the day wearing nothing but a diaper. A bit later he asked me what I was doing, and I think by the way he spoke he may have some developmental issues, as I could understand his 3 year old brother better than I could him.
I wore diapers for bed wetting until I was 10, and had no problem wearing openly around my parents, but I only ever rarely wore during the day, and by the time I was five I would never have let anyone apart from my folks see me in my diapered state. This little fella had no such issues though, and happily sat on the floor eating, or playing in just his disposable.
Apart from myself at that age, he would be the oldest kid I've encountered wearing a day nappy.