This landed in my inbox this morning, indirectly from Web usability expert Jakob Nielsen:

Between the ages of 25 and 60, the time users need to complete website tasks increases by 0.8% per year.

In other words, a 40-year-old user will take 8% longer than a 30-year-old user to accomplish the same task. And a 50-year-old user will require an additional 8% more time. (Mathematically inclined readers will note that this increase is linear, not exponential.)

Nielsen explains the gap as follows:

Two factors cause the 0.8% increase in task time. For each additional year of age, users: spend 0.5% more time on each page and visit 0.3% more pages per task.

In other words, the biggest factor is that older users need more time to understand pages, scan the text, and extract the information. A smaller — but still substantial — problem is that people have more trouble navigating websites as they age. It's not surprising that users need more time to use websites as they age, even within the mainstream group of 25–60 year olds.

The human aging process starts around age 25 and causes erosion of cognitive resources, loss of visual acuity, degraded reaction times, and reduced dexterity. People need more time for the same mental operations; they have less memory capacity and take longer to process the same perceptual input. All of these elements of human performance impact the speed with which users can get something done on a website.

There's also a coverient: the age at which people started using the Web. Because the Web is relatively new, a 50-year-old might have started using it at age 40, whereas a 30-year-old might have started at age 20. In contrast, by 2050, a 50-year-old will have used the Web since age 5, and thus benefit from 45 years of experience. A 30-year-old user in 2050 will have only 25 years' Web experience. This added experience might eventually allow older users to catch up and somewhat reduce the 0.8% gap. Although we obviously can't predict the future, my guess is that the age penalty will drop to around 0.5%/year. Still, this doesn't matter much for your Web strategy over the next 10 years: the 0.8% level is where we're at and where we'll remain for some time.