I don't really see the problem - if they're all happy, then it's all good... It's kinda harsh to tell a young child they can't have a certain kind of comforting anymore, no? We all grow up and move on at our own pace. Some kids suck their thumbs well into primary school (I know a certain young lady who had to be weaned from her thumb at the cusp of high school).
I guess I just don't see the harm.
Well said.

My sister had her "sooky" (a horrible rag that she used to suck) until she was 12.