I've had a quick rummage using the search function and found this:
Hi,i’m currently 15 years 6months old and i’ve been wanting to be a marine for sometime now,I was planning on going to AFC harrogate first then joining the marines.After hearing people say there’s simply no point in doing this,Is the best option just to outright join the marines at 16?
www.royalmarines.uk
If you dig about you'll find other specific stuff.
From memory, the 'sweet spot' statistically for successfully completing RT is about age twenty-one. But this doesn't mean that youngsters (or oldsters) don't do it.
I joined another service shortly after my sixteenth birthday. Having recently passed my
sixtieth birthday I'd suggest keeping an open mind and enjoy stepping out into the independence which later teen years afford.
Physically you will radically change over the next five years plus. This is a physiological certainty. If you have a career path in mind you can perhaps consolidate and build upon this by living a healthy life (no fun at all) and putting on some running shoes. Best of luck. Let us know how you get on.