A quick addition, based on my actual remembered experiences of being nappied.
As I mentioned, I was diapered up until late second grade (I thought it was mid-second grade but I know I was 8, which wasn't until late second grade).
Obviously, there are a ton of photos of me from kindergarten. In the photos taken from late '96 to late '97 (2 to 3 years old) I'm rocking the t-shirt and diaper look a lot because, you know, that's what 2-year-olds do. In the photos taken from late '97 to mid '98 (3yo), I've got the 'proper little man' look going on - polo shirts, shorts, riptape sneakers and all.
Around June '98 (which is, I'm informed, shortly after my family stopped trying to push toilet training me, and decided it would happen when it happened) I apparently sharply regressed - or the way my teachers treated me regressed, at any rate. It's very noticeable because in my '97 class photo (around Christmas), I'm dressed nicely, but in my '98 class photo (almost exactly a year later) I'm wearing one-piece pyjamas with a print. Nothing beats my '99 class photo, though - I am perched at the end of the back row, IN A T-SHIRT AND DIAPER, two months after turning 5. I know that they basically called us up in what we were wearing, but that's a real mind trip.
From June '98 to Dec. '99 in general, though, I seem to have given up on pants. I checked and there are like a single-digit / low double-digit number of photos in which I'm wearing pants, mostly my Sunday best or other really nice occasions. In all the other photos, I'm wearing a t-shirt or robe, sometimes shoes and socks - but never pants. A vast variety of diapers (normal diapers, overnights, swim diapers - never Pull-Ups or Goodnites), though.
I think the most eye-catching photo from this time (apart from my '99 class photo) is one specific photo in which I'm with a group of kids my own age (~4-5yo) who are dressed normally, and I'm wearing a diaper and socks, and using a pacifier. I often stick out of groups these days (I'm tall and dark and have a face that could be diplomatically described as rugged) but never so much as in that photo.