I've already gone through the whole adolescent lazy hygiene phase, but with my youngest (12), it seems to be something different ... and far more frustrating. While he has dysgraphia, which affects fine motor skills, I don't think this falls under that umbrella.
I'm afraid I don't have any helpful advice, but fwiw we've had very similar experiences with our dysgraphic/DCD ds12. When he was younger he had a really tough time with changing his clothes, and it was always a huge battle. It was easy to see that it was difficult for him to do due to his fine motor challenges, but for him it morphed from something that was simply really tough to do into... "why the heck do people need to change their clothes?".
He luckily doesn't have trouble with plaque buildup, but we have to remind him every single night of the year to brush his teeth before he goes to bed and after breakfast. I've given up reminding at other times at this point to try to minimize the reminding and work on (hopefully) building automaticity morning and night... but honestly... if it hasn't happened by 12, not sure it's going to happen! It also seems that he's developed some of that same "Why do this?" line of thinking that he fell into earlier re changing clothes every day.
As you noted, this is *so* way beyond different than our experiences with our other non-dysgraphic non-DCD kiddos - my girls just do it without thinking about it.
polarbear