For the same reason that the military doesn’t draft older men as soldiers, we’re a whole lot less into the drama of unlimited liability and the romance of dying and we don’t happily take correction from a 20-year-old with a stupid haircut.
I’m 48, 30 years ago, I did give the military serious consideration and I just did a quick then-and-now Q&A in my head and I’ve gone from responding to commands with a snappy “Yes sir, right away sir” to an all-purpose “You want me to do WHAT? Who are you, my wife?” We make terrible soldiers uniformed or not, and if you have what it takes to be a suicide bomber, middle age isn’t a concern, generally speaking.
Interestingly I read about a study the Nazis…errr the Strategic Rocket Forces… USAF, basically an organization dealing with mass killing did. In it they found while young men made good footsoldiers and junior leaders, they were a lot less affably bloodthirsty than older married men. Sure young men would willingly undertake actions like flying into dense AAA fire to drop bombs on a city. But the would balk at the prospect of actually coldbloodedly killing the enemy on a large scale. Single, young men, even those who were removed from the pointy end, were uncomfortable with making the attack plans. You kindly, middle aged surburban house owner? He would sign off on the destruction of a whole continent without as much as a second look.