Every nation has its own standards. Without a specific country, it's difficult to answer meaningfully. As a general rule, most countries allow any of their citizens who meet age, medical, background...
women should be able to serve in the military because they are becoming more and more powerful in every other aspect of life, why not in the military? As a military hopeful myself, I see no reason...
Traditionally, from the beginning of time (mankind), until the end of the Vietnam War; the abled bodied males were conscripted into service. Examples: Troy, Sparta, Rome, Attilla the Hun, Ghengis...