Don't mention it!!!!!!!!!!!!!!!!!!!!
Well at least, I would rather cut my arm off then put something so negative (which they will perceive it as, if you keep dwelling on it and reminding them!) in my personal statement where you basically -
1, have a limited amount of room ANYWAY
2, should be full of positive stuff (it's like putting in a personal ad about yourself I guess?)
3, should focus on why you should get an interview/offer (i.e. all your work experience, voluntary work etc).
If you really want to 'explain' yourself, get your reference to stick it in somewhere and for them to say that they really think that you are a late bloomer and won't have academic difficulties with the course. If not, then send an additional letter with your application, to your unis, to explain.