regarding training at 16.
i think its very generalised that you shouldnt train hard at 16/17.
if what your saying is true, then the RM probably would have the minimum age for joining at 18.
only do swim training at 16? thats complete bollocks.
just because you had problems, doesnt mean all teenagers do.
just a few variables to consider:
family history of arthiritus, weight, height, musclular strength, running surface, frequency of running, intensity of running, running form, number of rest days, diet, where you live, alcohol, tobaco and drug use, daily calorie intake, equipement used (shoes etc), history of exercise, and also genetic make up are all factors that can cause injury or stress from exercise.
for example, you could have had poor running form, which would have cause impact related stress' on the legs and hips. that would be a problem fairly unrelated to your age, and is by no means a problem for all 16 year olds.
just think about all those young football players who are 17 and 18. rooney was an example.
they must have started playing football from an early age, and almost certainly played for youth teams before playing professionally.
all sorts of young athletes tend to have a long history of consistant training.
you do not get to play for england or compete at top level by doing swimming and light to moderate exercise.
i personally did serious exercise about 3 times a week while at school, usually a sport, cross country for a good 2-3 miles, and training for school teams. i had this sort of routine for a good few years, and have not had serious health issues.
teenagers reach physical maturity earlier then they used to, hell some 13 year olds can grow full beards nowadays.
at 16, most males have finished the vast majority of growth in terms of bone structure. unless your seriously weighty, run 3 miles 5 times a day everyday or enjoy taking a hammer to your legs, its quite hard to do long term structural damage.
getting fit at 16/17 means its easier to keep fit. if your body develops and forms around a fit body you will be structurally built in a way that lends itself to exercise.
your body adapts to its environment. sit on your arse until your 19, and you will most likely get a beer gut and poor cv fitness.
if your in good shape and are pretty fit, when it comes to 'bulking out' (ie stopping growing and producing muscle and/or fat and develop a 'male' physique), areas like intercostal muscles, legs, stomach, arms etc etc will all develop, because there in use.
lots of people say training as a teenager is bad, but it isnt if your sensible.
the human body is an amazing machine. training at 16 is perfectly possible.
just a couple of examples of things to show how far you can actually go:
http://www.howtobefit.com/tendai-marathon-monks.htm
http://en.wikipedia.org/wiki/Ultraman_% ... allenge%29
only 46 people have done the first one, but that shows that a lot of stuff is just hype. this sort of stuff is basicly just like the people who make you wear gloves and goggles cutting your hedge, or who make you have a lecture on the fact that drinking car battery acid is bad, and if you drop a hammer on your foot it will hurt, before you can so much as think about repairing a car.
your in the wrong career if you so worried about long term health damage.
apart from the obvious risk of being shot, its well known the RM suffer premature arthritus, can have hearing loss in later life, PTSD, and a whole lot of problems related to spending prolonged periods of time carrying heavy bergens, spending lots of time exposed to gun fire and loud bangs and stuff life that.
im not doubting commando training takes its toll. if your 16 with only a small history of training behind you, yes, you may well struggle, but that is more to do with inexperience then young age.
and simply, most of the time, if you are prone to an injury because you train at a younger age, it will not resolve itself magically when you turn 18.
things like arthirtus will get you when you train hard, whether thats at 16 or 26
. 
