Generalities are dangerous. 18 year olds supposedly have the fastest reactions, but 18 year olds generally have not achieved their full upper body growth.
The real reason the military likes 18-19 year olds is because they are easier to condition. In other words 19 year olds will accept orders that 25 year olds would laugh at.
If the US Army of Vietnam War had had an average age of 25 like it did in WWII the response that Lt. Calley would have gotten to his orders at My Lai to shoot civilians would probably have been a refusal... if they didn't shoot him. Remember that it was an older helicopter pilot Warrant Officer who put an end to the massacre by telling Calley he would open fire on him if they shot any more civilians. Calley himself was a 22 year old OCS grad who would never have been commissioned during WWII or Korea.
I was often at more risk from the other bored, armed and unsupervised 19 year olds than I was from the NVA. I could tell stories about idiots playing catch with concussion grenades and blowing the shyt out of themselves.... blowing holes in their own boats.... getting splinters from blowing up ammo boxes to "see what would happen", etc etc.