My sister had four children and didn't believe in aggressive potty training. This was back in the '70's. Her kids were in diapers until they were about three years of age. She and her husband encouraged them to use the toilet, but didn't pressure them, and eventually they wanted to be more "grown up", so used the toilet.

My husband and I didn't believe in pressuring our kids, either, but I did actively potty train them. A time came when I was tired of dealing with diapers. My kids were around 18 months when fully toilet trained.

One reason to potty train a child is because they go to playgroups and preschool. There's an element of peer pressure to the whole thing...little kiddos see other people (parents, sibs, other little kids) use the toilet, so they want to do the same thing.