From what I was told at the AFCO last week, for the Royal Navy, you need to be on your first day of training no later then your 26th birthday. So you are able to start on your 26th birthday, just not after it. I may be wrong, but this is what the guy at the AFCO told me, as I'll be 25.5 when I finish my degree, so need to get my apps in at the start of my third year.
As far as I am aware for the RAF it is simaler but the age is 25. So no starting after your 25th birthday.