One thing I want to point out is that people complain about 14 and 15 year olds going out on journeys and how unrealistic it is when compared to 'real life.'
My brother left home at 14. Sure, he was a little brat for doing so, but he did it non-the-less. He had a hard time and learned a lot of big lessons about dealing with things that he wasn't then completely prepared for, but he eventually overcame those 'boss battles' with a bit of 'grinding'.
He's not the only teen or youth to leave home at a young age and battle their way into adulthood by themselves, and he probably won't be the last. So saying it's unrealistic for people that young to go out there and do battle in the world is just stupid. Perhaps they aren't slaying demons intent on taking over the world, but they are battling their own demons that are intent on destroying their world. Oh wait, same thing, different face.
I'd also like to point out that a good story teller can justify anything in their own world.
A seven year old with the power to destroy the cosmos? Perhaps an ancient demon is trapped in his body. Perhaps he is the son of a god/dess and has access to other-worldly powers because of it. Perhaps he fell in a magical well when a baby and absorbed it into his soul, slowly using it up as time goes by. Perhaps he has no idea how to access it, even in dire need, but still holds it inside. Perhaps it only comes out while he sleeps.
A twelve year old able to battle on equal footing with soldiers double her age? Maybe she's been trained since childhood, focusing on her strong points and working to perfect her few abilities. Maybe hers is a clan that absorbs the memory and skills of those they defeat. Maybe she was born under a lucky star and bumbles her way through battles with pure blind luck. Maybe she can read minds and knows where the enemy will move next. Maybe she fell in a cauldron filled with a potion of strength when a child and has since retained that strength.
>.>
<.<
I've said it more than once but I'll say it again - it doesn't matter how cliche a story is as long as the details are new. A rebellious teenage princess who runs away from home to aid the main character on his quest? Maybe she's cursed to become a flower when she sneezes and the hero has to keep running around finding her. Even worse if she has allergies. Maybe she is down-to-earth and always carries her own weight, being level-headed and reliable, always noticing the traps and sardonically pointing out the cliches and tropes that pop up from time to time, lampshading them. Perhaps she is really a he, and he wanted to escape an upcoming marriage by taking one last adventure, so pretended to be a girl when meeting the hero for the first time - who thought he was gonna hook up with the pretty princess at the end of the game~
They start at the age of 7 to become knights. That's a lot of time to bulk up.
But hey, this is why we make fantasy RPGs if that -isn't plausible-.
Also, since we went there, which armor are we talking about? Since I'm pretty sure that armor shape revolved depending on maneuverability and some such and not exactly about your muscle strength. They went through various revisions before settling with the Milan Armor in Renaissance. And even before, I don't recall seeing armors catering only to a specific body type. Armors were rare since tailoring an armor to the Knight's needs is actually super expensive, so a lot of times their equipment is incomplete (Like people barely have helmets). And sword and shield usage is actually super dependent. I can see a 13 year old being able to do it based on the training we had for our massive Troy play.
And if we're going with poverty stricken countries, I'm sure as hell that Mindanao didn't need an armor to fight back against the Spaniards/American/Japanese and their guns. They utilized the environment instead. So at least there's still that route.
Yeah, this is why Leather armour is such an early-game staple in most RPGs - it's more accurate for people to have worn cured leathers and hide en masse for armour than actual metal plating and the like. A lot of it was tailor-made for a specific person (usually nobles, important people and higher-up knights), not churned out by the bucket-load for every Tom, Dick and Harry to try on. Hence why in traditional RPGs it's so costly, scarce and end-game gear. There may not be much Traditional RPGs get right, but there was some thought put into item cost, availability and the like~
Also, armour wasn't as rigid as most people think. Truly well-tailored armour like those knights wore, were very easy to move in and non-restrictive because, well, when you're swinging big-ass swords around you don't want to be hampered in any way with your back-swing and movement. Heavy, yes. Hot, yes. Restrictive? Not so much when made well.