What is the difference between humans and mutants in X-Men? Are mutants born to normal humans? Also, how do mutants get their powers? Are they born with them?
A quote from Marvel Universe,
Human mutants are considered to be of the subspecies Homo sapiens superior or simply Homo superior, an evolutionary progeny of Homo sapiens, and are considered the next stage in human evolution, though whether this is true or not is a subject of much debate in-universe. Unlike Marvel's mutates, which are characters who develop their powers only after exposure to outside stimuli or energies (such as the Hulk, Spider-Man, The Fantastic Four, and Absorbing Man), mutants are those whose mutations are pre-natal, and whose powers typically manifest at puberty.
So going off that, mutants are a subspecies Homo sapiens superior that manisfest during puberty. Rather than those who have become a mutants via outside stimuli or energies.