Final answer:
Juan is likely at least 2 years old, as that's when children typically see a significant increase in vocabulary, sometimes going from 50 to 400 words as part of the 'vocabulary spurt' that occurs in early childhood.
Step-by-step explanation:
If Juan's vocabulary suddenly jumps from 50 words to 400 words, he is probably at least 2 years old. By the age of 2, a toddler typically uses between 50 and 200 words, but by 3 years old, children generally experience a significant increase in vocabulary, having up to 1,000 words and starting to speak in full sentences. The rapid pace with which vocabulary increases during early childhood is sometimes referred to as a "vocabulary spurt".
An expansion in vocabulary at a rate of 10-20 new words per week is common, and by 5 years old, children understand about 6,000 words and speak 2,000 words. At this stage, kids also start to define words and question their meanings. Therefore, seeing such a vocabulary jump indicates that Juan is likely navigating the early stages of language acquisition and is in the toddler age range.