While people who have reached advanced ages have always been aged one, two or three at some point, the "toddler" is a recent creation.
The word itself can only be dated to the late eighteenth century, and the OED provides a quotation from 1876 that refers to toddlers of age six or seven!
It's not until around 1930 that the word came into common use in the trade press for clothing makers and clothes stores, and the late 1930s that it was in common use.
One of the interesting things I have learned in The Commodification of Childhood.