That happens quite a bit, but it could have been to dodge any possible backlash. Fourteen year olds in ANY war scenario are generally frowned upon.
But then why not just make her look and behave like an adult as well as supposedly being one? And Jack is explicitly confirmed to have previously been a child soldier in a cutscene very near the endgame, so it's not like they haven't at least brought up the idea.
I think, and this is just opinion here, that it might have something to do with Otacon/Hal showing up again. Obviously the character history between them (he slept with her (his step-)mother, while their father drowned which Emma was watching) might be reminding her of those times, and that's what the writers were thinking when they thought up the mannerisms of her character.
Or I might just be being ridiculously defensive of the game because it's one of my favourite series of ever. Even though MGS2 is my least favourite of the games so far.
Quite a few of the characters are alleged to have been child soldiers in that series. Frank Jaeger/Grey Fox/Ninja/
in... several games. Raiden, like you say, Mistral from Rising and Drebin from the fourth one. Chico in Peace Walker is probably the only actual child soldier though. Then there's all the children that Big Boss saved way back in Metal Gear 2, though they weren't actual soldiers, afaik.