If you take sex as a purely physical non-communicative act then the younger (approx 17-25 )the better as they are going to be in the best shape of their lives. The female peak ages for reproduction are 19-24 and therefore a normal male will be more attracted to a partner who is or resembles a woman of that age group.
However, sex is a bit more subtle in the real world. I am of the opinion that the personality (humour, intelligence - emotional intelligence if you will) is the chief demand amongst everybody- not just punters. Now if the girls were to advertise their IQ / education as well as their age then I would bet a lot of money that the smarter girls would clean up.
Lets put it this way- Carol Vorderman or Lindsay Lohan.
By the way I voted for an 18yr old Carole Vorderman!