Over the last year (October 2006-October 2007) the share of the UK internet population made up by under 25-year-olds has decreased from 29% to 25%, while the share of over 55-year-olds has risen from 16% to 19%.
Overall, the average age of the UK internet population has risen from 35.7 to 37.9 in this period.
The site with the youngest UK online audience is online games portal Miniclip, with an average age of 28.1.
High street retailer Marks & Spencer has the oldest average online age at 46.5.
Five of the 10 online brands with the youngest average age are entertainment sites and four are social networking sites, among them Bebo, Nickelodeon Kids & Family Network and RockYou!
The five online brands with the oldest average age are all familiar high-street brands, including M&S, John Lewis and Nationwide.
Alex Burmaster, internet analyst at Nielsen Online, said that to see such large changes in the course of just a year showed that the internet population is undergoing "a significant ageing process".
He said: "When looking at how a particular audience is composed by age, a change in share -- even by just a few percentage points -- actually represents quite a fundamental shift.
"It will be very interesting to see whether this trend continues over the next 12 months and, if so, whether the types of services and products offered and marketed online adapt to reflect this changing population."