PEW INTERNET RESEARCH offers data gathered in the United States that says 18-24 year olds send or receive an average of 109.5 text messages per day.
That seems like a very high number to me. Further data puts the mean text point at 10 messages per day--more believeable.
In my Emerging Technology module, we will distinguish between the mean arithmetic average of a group of texting data and the median, the middle data point in a list of SMS activity. The median is the point at which half the scores are above and half the scores are below.
Download Americans and Text Messaging by Aaron Smith, Pew Internet Research, September 19, 2011.