The US military likes 18 year old soldiers because most of them are impressionable, easily manipulated or indoctrinated, they don’t typically have dependents to worry about yet, they are cheap to pay, and their bodies can take far more abuse before giving out.
Source: I was one.
It has absolutely nothing to do with reaction time with weapons. That’s Hollywood bullshit. You’d be surprised how infrequently you can even see the people you’re fighting in modern warfare.