The majority of 18-34 year olds in the US own a smartphone

"The majority of 25-34 and 18-24 year olds now own smartphones (64% and 53% respectively);
The majority of smartphone owners (62%) have downloaded apps on their devices and games are the top application category used in the past 30 days;
The number of smartphone subscribers using the mobile Internet has grown 45 percent since 2010;
87 percent of app downloaders (those who have downloaded an app in the past 30 days) have used deal-of-the-day websites like Groupon or Living Social;
Younger groups text the most. In Q3, teens 13-17 sent and received the most text messages (an average of 3,417 each month)."

Source:  Data from Nielsen, reported in NielsenWire, 15th December 2011