The shift by Americans to mobile devices has reached a milestone.

Fifty-six percent of U.S. adults own a smartphone of some type — up from 35% of adults two years ago, the Pew Research Center said in a report released Wednesday.

The report marks the first time in the two years that Pew has surveyed smartphone adoption that a majority of Americans say their mobile phone is a smartphone or that their phone operates on the iPhone, Android, Windows or BlackBerry platform.

The adoption comes amid a surge of shopping, surfing, social media and other activity on mobile phones in recent years. Twenty-eight percent of all mobile phone owners banked with their device in the past 12 months, up from 21% a year earlier, according to a survey released in April by the Federal Reserve Board.

Mobile banking had a high level of activity in April, according to an index compiled by American Banker, in terms of both new features offered by banks and customer adoption of those features.

Eighty-one percent of adults between the ages of 25 and 34 own a smartphone, although ownership also tends to be highest among people ages 18 to 24 (79%), 35 to 44 (69%) and 45 to 54 (55%), according to the Pew report.

Seventy-eight percent of Americans in households that have at least $75,000 in income annually own a smartphone, while 61% of smartphone owners earn at least $50,000 a year and 52% of owners earn at least $30,000 a year, Pew found.

"Younger adults — regardless of income — are very likely to be smartphone owners," Aaron Smith, a senior researcher at Pew, writes in the report. "Conversely, for older adults, smartphone ownership is more of an ‘elite’ phenomenon: smartphones tend to be quite prevalent at the upper end of the income distribution but much less common among those with lower income levels."

As for platforms, 28% of all cellphone owners use an Android device, an increase of 13 percentage points in two years, while iPhone owners represent 25% of smartphone-owning Americans, up from 10% in May 2011. The share of Americans who say they own a BlackBerry device has fallen six percentage points, to 4%, over the same period.

Platform choices vary by income and demographic group, according to Pew. Nearly half (49%) of cellphone owners in households that earn at least $150,000 a year say their phone is an iPhone, while smartphones that run Android grab a bigger share of the market than iPhone among owners with a household income of less than $75,000 annually.

African-American cellphone owners (42%) are more likely than whites (26%) or Latinos (27%) to say they own an Android device compared with an iPhone, Pew found.