ADHD is a developmental delay, so it makes since that things like potty-training are sometimes delayed also. This article mentions bedwetting in an interesting context to consider:
I know a family whose son (ADHD diagnosis) was still soiling multiple times a day in kindergarten and first grade. He was tested to be gluten-sensitive. When gluten was removed from his diet the soiling issue resolved immediately.
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism