By age 6, the average child has a vocabulary of around words

By age of 6, the average child has a vocabulary of around 14,000 words. Vocabulary development can be relatively variable among children of the same ages. Children typically speak their first word somewhere around 12 months. At 18 months, they may use around 50 words or lower and there is an increase in the number of words with age. 

When you are making a guess you are forming a hypothesis