Pseudo-Medieval fantasy settings are
notoriously loose about technological context. However this kind of armour was about as old as solid plate in general, seeing as how it was often made of damaged or just obsolete cuirasses (waste not want not right?), and
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were independently developed in Asia and used for a VERY long time.
You'd think that helmets were a no-brainer (ahem) but surprisingly many people skipped on them even in full battlefield harness. There's something to be said of unobstructed senses and breathing after all (which is also why open helmet designs were universally more popular than full-coverage ones).
OUT of battle basically nobody but guards on active duty wore the damn things because they're heavy and uncomfortable at the best of times.