But adoption varies greatly by age. Most Americans under 44 already carry a smartphone

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In the rapidly growing U.S. smartphone market, Google's (GOOG) Android and Apple's (AAPL) iPhone seem to have reached a steady state.

According to the third quarter Nielsen survey of the market, issued Thursday, 43% of Americans now own smartphones, up 5 percentage points in the past six months. Within that 43%, according to Nielsen, Android has 43% of the market and the iPhone has 28% -- exactly where they were three months ago.

What is changing is the rate at which different age groups are switching from dumb phones to smart ones. Young adults, not surprisingly, are more likely to carry a smartphone. Nielsen's stats:

• 62% of Americans aged 25-34 who own any kind of mobile phone own a smartphone.
• Among 18-24 and 35-44 year olds the smartphone penetration rate is hovering near 54%.
• Around 40% of teens 12-17 years old and 40% of 45-54 year-olds reported owning a smartphone
• The second fastest-growing smartphone penetration rate is those aged 55-64. Smartphone penetration among this older group is only 30%, but it jumped 5% this quarter.