Babies go through the developmental stages of talking during the first three years of life. By the age of 12 months, babies can usually say simple words like "mama" and "dada" with comprehension of what they are saying. At 18 months, babies often can say up to 10 simple words.
By 2 years old, kids can string together a couple of words to make phrases of two to four words. By the time a child is 3, he has a rapidly expanding vocabulary and can understand abstract and symbolic words such as "now," as well as feelings such as "happy" or "sad."