They label them this way, because often the males and females in "juvenile" plumage are identical (to us, at least) or so similar as to be indistinguishable from each other, or from adult females. But no, they do not become male later - they moult (I don't know when, for hummingbirds) and then the sexual distinctions becomes apparent. At least in Ruby-throated - not all species of hummingbirds actually have distinct male/female plumages, although I think all our North American ones do.
(Not a dumb question though - if all animals are being considered. Some species of fish are entirely born female, and only turn into males later in life, in response to changing circumstances.)
It was the =Calliope that most puzzled me, actually.
Not mean, just territorial. Sugar sources (whether they be flowers or your feeder) are a limited resource, the bird is just doing its best to survive by defending its territory.
Peter