Smartphone penetration in the U.S. has not reached that psychologically significant tipping point of 50% quite yet, but among young adults, the needle passed the midway mark long ago. Nielsen’s latest quarterly report on the mobile landscape shows that 62% of U.S. cell phone owners ages 25 to 34 have a phone with an advanced operating system.

Overall, smartphone penetration is at 43%, but a majority of people in the most desirable marketing demos now are running Android, iOS, BlackBerry or Windows OSes. In the college-age segment (18-24), 54% own smartphones, as do 53% of 35- to-44-year olds. This means that well over 50% of the 18-44 segment are now on smartphones and can be reached via apps or mobile Web. The dropoff in smartphone use after age 44 is sharp, however, with only 39% of the 45-54 segment on advanced devices -- almost on par with the 38% of teens (13-17).

In terms of OS share, Google Android continues its dominance of sheer reach, with 43% of the smartphone market over 28% for Apple’s iOS running on iPhone. Adding in other iOS devices like iPads and iPod Touch units would change that figure substantially, but when it comes to the most mobile device of all -- the phone -- Google’s share is formidable. Research in Motion’s historic decline continues from market share leadership only a few years ago to a mere 18% of the smartphone OS field now. Windows Mobile has only 7% of the market. But Microsoft’s recent partnership with Nokia and a version upgrade that will be pushed in new phones coming to market may give the also-ran another shot at the market in the next quarter.

Smartphone ownership remains the best predictor of mobile media use. Much as always-on broadband penetration reached a tipping point in the mid 2000s and sparked a sharp rise in online video use, social network membership and digital downloading, advanced phone technology invites new classes of mobile media interactivity. The dramatic increases in app downloading, mobile Web access, social network access, search activity and even ad inventory have paralleled the increased distribution of smart devices as owners seem eager to move many Web operations off their desktop and into their pockets.