According to a report from Nielsen,   the largest number of visitors to are in the 35-49 category.

Twitter Most Popular Among Working Adults

Twitterers (a.k.a. Tweeters) are not primarily teens or college students as you might expect. In fact, in February the largest age group on Twitter was 35-49; with nearly 3 million unique visitors, comprising almost 42 percent of the site’s audience. We found that the majority of people visit while at work, with 62 percent of the combo unique audience accessing the site from work only versus 35 percent that accessed it from home only.

Unique Visitors to by Age Demographic

Age Group Unique Audience Composition %
2-17 250,000 3.6
18 – 24 ** **
25 – 34 1,379,000 19.6
35 – 49 2,935,000 41.7
55+ 1,165,000 16.6
65+ 477,000 6.8

source: Nielsen NetView, 2/09, U.S., Home and Work
**These demographics have insufficient sample sizes

My only concern for this data is that I’m not sure what “insufficient sample sizes” means. I’d be curious how the 18-24 category compares, as that’s the demographic that academic librarians are trying to reach.   But from the data that is presented, Twitter does seem to be used primarily by the older (i.e., not college kids) crowd.