Hmm, Soldiers are adults, but they usually have mandatory PE. But I suppose they are volunteers. If only our students weren't conscripted into their educations.
Obviously, the requirements of being a soldier are comparable to the requirements of being a student, so it makes perfect sense if both have mandatory PE. Or not.
The requirements for being a soldier are determined entirely by the institutions which select people to be soldiers. So you'd imagine that if a school required it's students to do PE, they'd have just as strong of requirements as soldiers do.