Our son was three and a half and still in diapers.  Plus I had an 8 month old baby in diapers, of course.

My mom and my in laws and our older siblings were so critical of us.  It wasn't like I hadn't tried everything--Geez!

But he absolutely was NOT interested until he started pre-school and realized he was the only one still in diapers.  He finally decided it was time.