From my experiences with the RAF Careers people, I was told that you had start IOT prior to your 24th birthday (ie 23 or younger as Mike said). This is also written down on the most recent careers data leaflet about RAF pilots (double sided non glossy piece of A4 paper published Oct 2003).
You may encounter people at the front desk who will tell you differently regarding the age criteria so you'll need to be persistent but polite as well.