alt Hacker Newsdiminishing returns. people over 40 heal less quickly, start to run into chronic health issues, and are more likely to have suffered permanent injuries. it's easier to set a global cutoff at an age where the probability that any given person will be unable to do the job safely than it is to assess each person individually.
I wouldn't survive a week at the front just because of my back. But I'll happily catch a couple of bullets.