Twitter’s getting younger – well Twitter use, anyway. Twitter use by US young adults (18-24) “increased dramatically over the last year, with nearly one in three (31%) now using Twitter,” Pew Internet reports, up from 18% a year ago and 16% in late 2010. “The proportion of young Internet users who use Twitter on a typical day also doubled over the last year,” Pew found – to 20%. The research group sees a tight correlation between Twitter use and smartphone adoption: “One in five smartphone owners (20%) are Twitter users, with 13% using the service on a typical day.” It adds that non-smartphone users (people who use what are called basic or “feature” phones) “are roughly half as likely to use Twitter overall (9%). I blogged about how teen Twitter use doubled in 2010 and ’11 a couple of months ago here.