But 55- to 64-year olds who make more than $100,000 a year are big buyers too.

"Whether or not you have a smartphone is closely related to both how old you are and how much money you make," finds a Nielsen survey of 20,000 Americans with mobile phones conducted in January. I quote:

While overall smartphone penetration stood at 48 percent in January, those in the 24-34 age group showed the greatest proportion of smartphone ownership, with 66 percent saying they had a smartphone. In the same age group, 8 of 10 of those that had gotten a new device in the last three months chose a smartphone. Among those who chose a device in the last three months, more than half of those under 65 had chosen a smartphone.

No big surprises there. The twist in the report issued Monday is that when age and income are both taken into account, older subscribers with higher incomes are more likely to pack a smartphone. For example, those 55-64 making over 100K a year are almost as likely to carry a smartphone as those in the 35-44 age bracket making 35-75K per year.

In Nielsen's most recent survey of smartphone operating system shares, conducted in November before the bulk of iPhone 4S sales, Apple (AAPL) was the No. 1 manufacturer  with 28.3% of the U.S. market, but Google's (GOOG) Android had the largest market share (42.8%). See chart below.