Im finding more at 18th's and 21st's that the young kids are into Rock / Indie.
New artists such as:
Kaiser Chiefs (Oh My God, I predict a riot)
Killers (Somebody Told me, Mr Brightside)
Kasabian (Cutt Off, LSF)
The Strokes (Last Night, Juicebox)
Futureheads (Hounds of Love)
Franz Ferdinand (Take Me out, Do you wanna)
At my most recent 18th I was asked only to play Indie - and no R&B. I just got by as my collection is lagely R&B.
It's likely that if the youngsters are into their indie and rock, they really hate R&B and Urban - so don't assume they'll like "Your Humps", "Dr Dre" etc

It would kick off if you played those tracks.
Cross-over artists such as The Prodigy, Dodgy, and Underworld could also feature, but the average 18 year old is more musically knowledgable these days, so they also appreciate The Clash, The Knack, Beatles and so on.
Come prepared with a collection to suite Indie Kids, the Urban lot, and Cheesey fans.
Hope that helps a bit.