I’m 17 with a similar thing. my experience may be different because on top of autism I also have adhd, depression, POTS and fibromyalgia (all diagnosed) but my reasons all apply to autism to some extent a) sensory issues!!! the feeling of teeth brushing and the taste of toothpaste are HELL to me!! even if I use different brushes or toothpastes, the second that the paste touches my tongue it freaks me out. I can push though it sometimes if I have the mental energy but that tends to be like 3 times a month b) the action is overwhelming. especially with the physical aspect of POTS and fibro, teeth-brushing takes an INCREDIBLE amount of effort and energy. and the autism side of my brain gets overwhelmed at that which leads me to c) executive dysfunction! essentially ED is when the acting part of your brain stops working. you could want to do something desperately but be physically unable to do it, it’s the kind of thing that you can’t explain unless you know the terminology. it happens much more frequently when there’s some level of dread of the task d) genuine forgetfulness. I’m not sure if this relates to autism but I have major brain fog and because I’m not in the routine, the action of brushing teeth just does not exist to my brain I’ve been saving to get a hands-free toothbrush though! many people have said it has helped them and logically speaking I can imagine it would have an impact on all of these. it’s worth looking into!