This is the early answer:
Voter engagement and young people (July 2002)
"Research by the MORI Social Research Institute at the time of the 2001 general election suggested that low turnout was particularly pronounced among young people, with an estimated 39% of 18-24 year olds casting a vote."
and further details can be found
here.
Later research suggested it was even lower, with
this article from the BBC, reporting the Economic and Social Research Council, putting it at just over 21%.
The BBC piece finishes by comapring recent elections:
"
This was a substantial decline compared with .... 1997 - when it is estimated that 68% of 18 to 24 year olds cast their vote - and 1992, when 61% voted."
Hope this helps.