You could be forgiven for thinking Twitter was the latest example of youth culture.

From the ability to fire off grammatically-abbreviated updates about daily trifles to keeping tabs on celebrities, the fast-growing microblogging service has all the earmarks of a young person’s pastime.

But Twitter devotees are grayer than one might expect: The majority of Twitter’s roughly 10 million unique Web site visitors worldwide in February were 35 years old or older, according to comScore.

In the U.S, 10 percent of Twitter users were between 55 and 64, nearly the same amount of users as those between 18 and 24, which accounted for 10.6 percent of the total.

Twitter has seen its popularity explode in recent months, with the number of unique visitors to its site increasing by more than 1000 percent year-over-year in February, according to comScore.

Social media Web sites like MySpace and Facebook have also experienced an increase in older users recently. But the parade of elders came after younger users drove the initial surge in popularity (in Facebook’s case, of course, the service was initially limited to college students).

Twitter is a rare example of older people embracing a new Web technology at such an early stage, says Andrew Lipsman, director of industry analysis at comScore.

He posits that the knowledge to understand and use a service like Twitter is no longer confined to youngsters as a greater portion of the population has gotten older using the Internet in the past 15 years.

And he notes that social media services like Twitter and Facebook are becoming more and more entwined with business.

Twitter may even be catching on among people who have a reached a post-business phase of their lives: Of the 4 million U.S. Twitter users in February, 5.2 percent were 65 or older.