No shit because conscript mass armies and young men are both the physically best suited for war and, put bluntly, most expendable for their communities as they have always been. The age patterns were (and are)... quite different on the leadership level though.Many of the soldiers fighting and dying in the world wars were in their early 20s.
And historically as a rule unless men (women generally need not apply, though there were exceptions) were at least aristocracy all of them were the effective equivalents of buck privates at best. Quite a few warlike cultures started fielding men in diverse support and light combat roles already in their early-mid teens, but with very rare exceptions only began considering them as worthy of note and real respect many years later. Hell some considered slaying a foeman in battle the minimum requirement for being considered an adult - as you might imagine in such contexts becoming noteworthy took a whole lot more...
Plus, unless you were benefiting from family largesse achieving the kind of wealth needed to support a senior warrior never mind leadership position took a fair while. And indeed the reason for most men in premodern societies marrying fairly late was similar - developing the kind of career and income (or inheriting the farm, etc.) needed to support a family and duly be actually taken seriously as a suitor didn't happen overnight, a pattern still very noticeable among eg. career officers in the late 1800s.
Jeanne was also about as gigantic an exception to almost every norm of her day as you could find, and neither a fighter nor commander herself anyway. She was essentially a living standard, an inspiring rallying point, and her importance came from having been able to convince the people who mattered (ie. the King and sundry) of her usefulness, and in practice a major reason she got a hearing in the first place was the French (or more accurately,Joan of Arc was 19 when she died.
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) strategic situation was desperate enough that Charles VII was willing to try just about anything.You'd make a better case with say Edward of Wales, the Black Prince, who commanded the English right wing at Crécy aged sixteen, but then *he* was royalty so...