The decreasing numbers come despite the move to digital, which has increased the number of channels in the market.
"Younger viewers have been lost in disproportionately large numbers," said the report. "When they switch from analogue to digital they show an increased tendency also to switch away from television."
The BBC website was among those making gains as it saw its online audience rise by 3.1 million to 13.7 million a month.
However, the report found that BBC1 and BBC2 saw a drop in viewers over the past year because of competition from new digital channels.
BBC director general Mark Thompson promised to target the younger audience and teenagers in particular.
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