You may start your application to join the services from the age of 15 years & 9 months with your parent/guardian's permission. No permission, no go.
You MAY NOT join the service until after the last GCSE for your year 11 group has been completed. (Usually around June 30th). It does not matter whether the individual is taking GCSE's, is suspended, expelled or considers themselves to have left school, you cannot join before the end of year 11 for your age group.
Most would be advised to start the selection process sooner rather than later, when eligible, to avoid a long wait (sometimes up to 2 years) after finishing education.
And yes, it has been asked before by Chico & I see a contradiction on the horizon;
Hey, i was joining the royal navy to those who remember but i cant because my mum will not let me.
They're sixth formers so 16-18ish (entry requirement is actually between 15 and 17.5 in the September of year of entry). Candidates are 15/16ish when they do the service selection boards. My lad is there as a crab student in his second year and I went there when it was just army many many moons ago.