“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,” Alex Burmaster, Nielsen Online Internet analyst said in a statement. “Age compositions tend to evolve subtly over a number of years, so to see such large changes in the course of just a year shows that the Internet populating is undergoing a significant aging process.”

Burmaster said if the trend continues over the next 12 months, it will be noteworthy to look at “whether the types of services and products offered and marketed online adapt to reflect this changing population. New online offerings and technology are usually targeted at the young, but it's possible brands could be missing a trick if they continue down this path in the future.”

Nielsen also looked at brands with the youngest and oldest online audiences. Online games portal MiniClip had the youngest average user age, 28.1-years-old. Retailer Marks & Spencer currently has the oldest online average age, at 46.5.

Five of 10 online brands with the youngest average age have to do with entertainment, and four of them are about social networking. The five online brands with the oldest average age are familiar “High-Street” brands, the Nielsen report concluded.