Who does all the text messaging? Young adults by far.
The Pew Internet and American Life Project published the results of their texting study, showing that young adults text more than anyone else. The report refers to a lot of averages across demographics, but it seems that there were a lot of heavy texters driving up those averages. The medians are a lot lower. The chart above shows the latter.
Even the median for young adults is still high though relative to other groups.
At 29, I'm right at the edge of that young adult group, and I text maybe once or twice a month on average. Forty per day seems outrageously high. Kids these days.