• by Mark Walsh  @markfwal, September 10, 2012

Ahead of the iPhone 5 unveiling Wednesday, Nielsen on Monday said 55.5% of U.S. mobile users own a smartphone as of July, up from 41% in the year-earlier period.

Nielsen tends to be more aggressive than other research firms with its estimate of smartphone penetration, so that figure is probably among the highest out there. For instance, comScore puts smartphone ownership at 49% of the U.S. mobile market as of July.

That said, Nielsen found young adults are most likely to own smartphones, while teens had the largest increase in smartphone adoption over the last year.

Here are some of the key metrics from the Nielsen blog post today:

-The majority of American teens (58%) now own smartphones, growing from roughly a third (36%) of teens owning a smartphones in 2011.

-74% of 25-34 year olds now own smartphones, making this age group the highest among smartphone penetration in the US.

 -Android handsets continue to lead the smartphone market, with both a majority of smartphone owners (51.9%) and recent acquirers (58.6%) who obtained their handset within the last 3 months.

-Apple's iPhone followed with a 34% of smartphone owners, while Blackberry handsets held an 8 percent share of the US smartphone market.