We left DD fairly late before we bothered with potty training (she was about 3.5) and it was a piece of cake, she did it in a weekend and was dry at night within a week - easiest parenting thing we've ever done! So I was intending to do the same with DS (currently 20 months) but I'm suddenly wondering if there is any disadvantage to leaving them late, aside from environmental reasons. I can't think why there would be, but it's been a few years since I've done this!