Most people generally consider a toddler to be a child between one and three years old. However, some experts believe that a toddler can be defined as a child between one and four years old. So, in answer to your question, 12-18 months could technically be considered the toddler stage.
During the toddler stage, children learn how to walk and talk, and begin to develop their own unique personality. This is an exciting time for both parents and children alike! Toddlers are also notoriously active and inquisitive, so it's important to keep an eye on them at all times!