stage 1) infant. The child is born, and called a newborn for a few weeks, but then is called a baby. This stage is generally from age 0 - 18 months. There is some flexibility over when a child moves from one stage to another, but by 18 months, nearly all babies are toddling.
Stage 2) toddler. The child may be taking first wobbly steps, or walking well. The toddler stage is generally from 18 months- 36 months. The toddler goes from speaking only a few words, to developing a larger vocabulary, even using two and three word sentences.
Stage 3) Preschooler. This term is supposed to mean a child too young for school, but now of course we have Preschools for the preschooler. This stage is from 3-5 years, generally speaking.
Stage 4) School age, primary, then upper elementary, then middle school, then high school, etc.
So to answer your question, when your toddler turns three, you can call him a "Preschooler". .
at 12:58 AM on Jul. 11, 2011