Virgin have had some special offers just recently when teen prices are available to all children under 16. I think the latest date is currently 1st July. My sister in law was a bit frustrated as when she booked their flights for next Aug for my 2 nephews of 15 and 13 the company originally offered teen fares at £399 instead of adult at £699 but then (after the booking had been taken) said they had been stopped so had to pay full whack.

As Mizzy said Charter airlines usually set a price per seat and regardless of age that is what you pay.