There's a few more (pertaining to end of FTRS/mob service etc) but these are the most relevant;
Annually when not FD.
Within 8 weeks of following birthdays - 30, 350 40, 45, 50 and 2 yearly afterwards.
TA Re-engagement - when you apply to re-engage you are to be medically examined only if the CO demands and if the first two points are not fufilled (speaky engleesh?)
I believe you used to have to have a medical every time you re-engaged (ie used to be every 3 or 4 years) however with the introduction of the new procedures for Gp A engagements that was dropped to be more in line with the regs (to save medical cash no doubt!)