The adults do repeatedly send Kirito back into dangerous cyber-wars in the later books/seasons, it becomes his job because he's like the world's foremost cyber-melee-combat expert.

And in Naruto the adults send kids as young as 12 on dangerous military operations because once you get your headband you're pretty much considered an adult and a full soldier with all that entails. And this is a criticism, it's not an accidental worldbuilding artifact. Which makes it all the more frustrating that despite becoming a god and basically the ruler of the world, Naruto doesn't change this status quo.