Different for all of them. For DD I'd have said she was a child from about 18 months, she was so advanced.
For the boys, a bit older.
DS2 is 3 now and I post about him in the toddler section, but truthfully I haven't thought of him as being a toddler for absolutely ages now. But he's in nursery, and that seems to be where nursery-related threads come up.