A majority of American adults who have cell phones are now using them to go online, according to a study out today from the Pew Internet & American Life Project. And, within that group, 31 percent of those adults use their phones for the majority of their Internet access.

But the numbers get even more striking when you look at younger demographics. Among adults 18-29 who use the Internet on their phones, a remarkable 45 percent do most of their web surfing there. And for all age groups, preferences are shifting away from desktops and laptops and toward mobile devices.

(Need any more evidence you need to make sure that mobile version of your website looks good and works well?)

Pew’s most recent data comes from a three-week phone survey of 2,254 adults that concluded in April, but the center first began tracking the percentage of people who use cell phones to go online in April 2009.

“In the space of three years, we’ve seen the proportion of cell owners who do this almost double,” Pew senior research specialist Aaron Smith told. “Depending on where you start the clock on the consumer smartphone revolution — most people do that with the introduction of the iPhone in June 2007 — within the space of five years we’ve gone from basically zero to half the country, with a sizable percentage using cell phones as their main source [to go online].”

Smith characterizes the reliance on mobile phones for Internet access as “not only very fast but widespread,” meaning that more people across age and socioeconomic demographics are increasingly using phones to go online.

The age breakdown: For adults 50 or older, 11 percent of cell Internet users use their phones for most of their access. For adults 30 to 49, it’s 29 percent. And, as mentioned above, it’s 45 percent for 18- to 29-year-olds. (Pew didn’t look at teens in this study, but you don’t need a researcher to tell you they like their phones.)

Two-thirds of those surveyed cited convenience — the chance to be online anywhere, and at any time — as the top reason for accessing the Internet by phone. A handful of responders credited cell phones with being easier to use, while others said phones were their only way to access the Internet from home. (Consistent with previous years’ findings, those with lower incomes and less education were more likely to report using cell phones for the majority of their Internet use.)

So what kinds of numbers can we expect three years from now? Cell phone ubiquity and more mobile Internet reliance seem clear. Smith says the data from April’s survey came as no surprise but he won’t speculate on what’s to come.

“One of the things that we have learned is there could be something that someone is developing in their garage tomorrow that could completely blow any projection we — or anyone else could make — completely out of the water,” he said.

Photo by Yutaka Tsutano used under a Creative Commons license.