The perception among younger adults is that everybody owns a smartphone. When numbers like 50% of U.S. cellphone owners have apps, the reaction inevitably comes, "only 50%?" It is easy for adults, say those from 25-44 years old, to forget that there is a significant portion of the U.S. population that does not own cellphones, let alone those of the smart variety. Mobile penetration in the United States is at 77%, which lags behind many other developed countries.

Nielsen came out with its third quarter mobile numbers today and the demographics are intriguing. The reason that young people feel like everybody has smartphones is because they do. 62% of people 25-34 years old have smartphones. Of all cellphones in the U.S., 43% of them are smart.

In terms of platform market share, not much has changed over the last several months. Android is still topping out around 43%, Apple gained a percentage point to 28% and Research In Motion lost a point to 18%. Windows Mobile CE is still hanging around (how we do not know, people just not upgrading their phones) with 7% while the "other" category that includes webOS and Windows Phone 7 dipped a point to 4%.

These numbers more or less mirror what comScore came out with in early October. Since the last time Nielsen released its numbers, the total percentage of smartphone users has risen from 40% to 43%. The mobile industry thought earlier this year that the inflection point for smartphone adoption, 50%, would be reached by the end of 2011 but that is not likely to be the case at this rate. The holiday shopping season should give smartphone adoption a boost and the major OEMs have devices lined up for the season, such as the Motorola Droid RAZR, Samsung Galaxy II and Nexus Prime and the iPhone 4S. BlackBerry and Windows Phone are not likely to see any appreciable gains this holiday season as the BBX platform from RIM and Windows Phone from Nokia will be released in the first quarter of 2012.

The demographic numbers are intriguing. Nearly one out of every five senior citizens has a smartphone at 18% while teens and their parents (the 13-17 and 45-54 demographics) hover around 38-39%. Check out the chart below.

Nielsen did not release numbers that segment platform among demographics but it would be interesting to know how many smartphone users in the 25-44 bracket use iOS or Android, for instance.

When and what is your next phone purchase? When do you think the U.S. will hit 50% smartphone penetration? Let us know in the comments.