Good question. So far as I'm aware the answer is yes, but the Official RM Website states rather ambiguously that you cannot be in full time education.
I'll query that one for you as that would also indicate that full time Uni students cannot join either, which is not the case. I rather suspect it means you cannot join before all GCSE's have been completed in your secondary school Year 11, even if you are 16 before that date. Or put simply, "not in compulsory full time education".
We have got lads in holding troop that are at college, one is definetly there full-time one is an apprentice with day release or something of the sort, from that I would say you were able to yes, but Ninja will confirm.