Quoting Kelly&Coralie:" Cora has never been one to put things in her mouth. Even as an infant, she never did.(which I thought ... [snip!] ... cannot be normal, can it? What can I do besides telling her over and over again not to do it and explaining the consequences?"
My four year old never did this as a baby or toddler...and now I find her doing it. It's weird. I explain to her that by putting stuff in her mouth she is putting germs in there and that's what makes her sick (feel yucky), then I just remind her everytime I catch her doing it. I think she just sees other kids (often younger) doing it and it becomes a weird habit.