Let's do what RWW did not and actually think about what this graph is showing. For each age group, their use of Twitter is plotted over time, relative to their use of the internet as a whole. In other words, this is a visualization of the relative composition of the Twitterverse. If all age groups used Twitter similarly to their overall internet consumption, then all the lines would be at 100.
I do find it amusing that RWW has a almost cliched "statistics can be misleading" section in its article, which fails to note the single most important caveat (unsurprisingly, given their misinterpretation of the chart): increased participation by any one age group must be offset by decreasing participation by another. So the rise in the "12-24" line is equally and exactly offset by declines in the adult groups. Kind of a different headline, isn't it: "Adults Abandon Twitter!" And yet, it's based on the exact same information.
At this time we should note that just two days ago, the Times ran an article called "Who's Driving Twitter's Popularity? Not Teens."
The key here is that we don't know whether teens are using Twitter more or adults are using it less. All we know is that if you look at the Twitter userbase, teenagers form a greater percent of the community than they used to - even though the absolute number of teenage Twitterers could be static or even dropping (if adult use was falling off at a greater rate).
What's much more interesting is that for the first time, teens are using Twitter disproportionately - they are a larger demographic of the Twitterverse than the internet generally. But again this gives us no context, and that fact could arise from their increased participation or adult accounts going stagnant.
It's interesting and informative to note that young people are a steadily growing percentage of the Twitterverse. It is a mistake to make assumptions about their number from the graph, however.
I fully expect an article from RWW examining the "massive rise" in "2-11" Tweets - who are these tweeting toddlers? What do they tweet about? And most importantly, how can your marketing strategy take advantage of this trend?
Update: I am not surprised to learn that this graph comes from Silicon Alley Insider's Chart of the Day column. I cringe at the thought of that site's influence.