First, I have to admire your move to put yourself in a volunteer position to help the war effort. And the effort needs cooks and mechanics!
I am 63 years and have seen a substantial decline in energy. I used to work the drilling rigs in Alberta. I recently watched a video of the workers "making a connection," a common task I did hundreds of times. Only two minutes, but lots of iron to move around. I can't do this work anymore. If I take on being soldier, I will be a liability on the battlefield.
So there is good reason for the 60 years as a max. While a few of us might be able to handle the job, many won't.
Anyways, all the best in your volunteer efforts wherever they may take you.