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CHANNEL SEVEN may have the largest audience of the commercial free-to-air networks but if its rival Channel Nine succeeds in luring younger viewers away it could also be the oldest.

Seven has disproportionately more viewers over the age of 55 than rivals Nine and Ten, according to an analysis of OzTAM viewing data.

Advertisers tend to chase younger viewers and eschew those over 55, leaving them one of the most unloved, some would say undervalued, audiences.

Nine is gnawing away at Seven's share of younger viewers using programs such as The Block and The Voice to lure them away from Seven's Dancing with the Stars and Australia's Got Talent.

In the current ratings period, Nine has just beaten Seven in its share of 25- to 54-year-olds - known by the industry as the money demographic because that is where the bulk of the $4 billion in TV ad dollars is directed.

''Nine has executed what appears to be a very effective 'wedge' strategy against Seven,'' the media analyst Steve Allen of Fusion Strategy said.

''They are taking away their younger viewers.''

Seven's greatest gains in primetime - the 6pm to midnight time slot - have been with older viewers, by a margin of between 2 and 3 per cent.

In the week ending May 26, the latest week for which complete figures are available, the only program Seven had in the top 10 for city-based viewers in the 25-to-54 age bracket was the US drama Revenge. However, Seven takes out seven of the top 10 shows for viewers 55 and over with Downton Abbey, Dancing with the Stars and Packed to the Rafters.

Mat Baxter of the media buying firm Universal McCann says if Seven  gains more older viewers and it becomes a trend,  then advertisers would be concerned. '' It's not yet enough to be a problem and  I don't think that Seven would be worried because it is within those levels of tolerance which is generally about plus or minus five per cent. If it goes over that then the market would start looking at that.''

Seven is confident that by the end of the ratings season in November it will still have the greatest number of viewers and will win with the age groups favoured by advertisers. It is also set to retain the greatest share of the $4 billion TV ad market.

Seven's chief sales and digital officer, Kurt Burnette, put the current rises in the age of its audience down to the programming cycle which, he said, would turn in its favour later in the year when Beauty and the Geek, the AFL grand final and X-Factor were on air. ''You can't generalise across the year. There is a programming cycle and it's very different from one quarter to the next.''

He acknowledged that the success of The Voice had taken Seven by surprise.