Teenagers use just 20 words for a third of their speech, according to a new study by Lancaster University.
Interesting, but not entirely surprising. - Probably partly down to the fact that big chunks of what we say is not about imparting meaning but rather just reaffirming social patterns and structures (from saying hello to complaining about the weather / the traffic / school etc).
On an aside a fantastic book by ethnographer Kate Fox called Watching the English looks at this kind of thing (though not from the teen angle specifically) while dissecting English habits and customs most entertainingly.
But back to teen slang, and more interesting to me is that while teens are using few words, slang is becoming more divergent from mainstream modes of English. Some forms are now regarded as genuinely unique dialects by researchers, where as in the past they were just modes of speech - ie not (nearly) sufficiently different to be a dialect. Good article and a bunch of definitions in The Independent here.
Anyway you can test yourself on your brit-teen slang on BBC News here.