Well, let me rephrase the question: what makes anything mature? Showing suffering, seeing blood and guts, letting the viewers know the weight of death?
Yeah, it can. But anyone can see blood and guts at any age, whether real or in a movie. That stuff is gross and shocking, but it’s a fact of life we can all see for ourselves. Yet if a kid were to see blood and guts, and have no lasting psychological problems, what would it matter? S/he’s going to go back to watching some age-appropriate TV show on a kid’s channel.
On the other hand, if that kid were to ruminate over what s/he saw, what might happen? An existential crisis? The themes that have to do with the loss of innocence will lead to maturity.
And it doesn’t take blood or guts to do this, it can take a life-changing thought. So then it doesn’t take blood or guts to make a story mature. All of the blood and guys is icing on the proverbial cake, so to speak.
Seeing dead bodies hanging from a ceiling? Ooh, scary. Like a campfire story. Seeing a teen anguish over having his hometown striped away, due to his father who died a violent death, and express conflict and insecurities about living up to his responsibilities…that’s where the maturity lies.
You see, it’s not just what you show, it’s how what you show makes you think. Couldn’t you do this with any of the Final Fantasy games? I suppose. But you can only write a story with so much in it so that it can only go far. It might shake up a kid. But it won’t get us to think about our existences, fears, flaws, broken hopes and dreams.