Just to show it's not simple, DS had fewer than 5 words in regular use at his 22 month check up! He was talking in 5 word sentences by 26 months, though; those 4 months in between were interesting.
DS12 developed similarly -- 15 words by 22 months 2 weeks; then one day he used 3 or 4 new words in the space of a few hours. Two weeks later at 23 months, he had an active vocabulary of 75 words.
Most were nouns; I don't recall him saying anything that sounded like a sentence then. But by the time he hit 32 months he was asking not just 'why' questions, but 'what would happen if' questions, e.g. 'What would happen if the earth doesn't (sic) revolve on its axis?'
