Miles,
I was just about to turn 23 when I started my application, have been accepted as pilot, and am starting IOT in May, 24 in October. My m8 is also in as a pilot, but he's starting in April since he's 24 in May, so if you come up to scratch, they can and will make sure your age is not an issue, so long as they have time.
As for acceptance rates, I was one of an OASC group of almost 50, of which well over 50% were going for pilot. After the first cut, there were only 3 of us left going for pilot, and as far as I know I'm the only one who got it. They run two OASCs of 35-50 people 48 weeks of the year.
What does that mean for your chances? Absolutely nothing! Be good enough, you'll get in. I was probably not the most amazing candidate on paper, but I had well thought out reasons behind everything I had ever done, and every life decision I had taken, and I have absolutely no doubt that the extra maturity and life experience from being about 5 years older than the average applicant helped immensely. You will almost certainly come across more confident and suited to leadership in the interaction elements, be that the interview, the group "classrooom" exercises, or the hangar exercises.
Best of luck to you!