This world look a lot like yours, if you except that magic does exist here.
Magic, a mysterious energy that, for some reason, have a tendency to accumulate into the body of humans, especially the females ones. To the point of forming what is called a magical-core, some sort of organic stone in the body of a magical-girl, created by their magic to let them stock said magic.
Most decide to use their powers to become celebrity, ‘idols’ as they're so often called, but those magical girls usually loose their powers upon reaching adulthood. But the ones using their powers to protect the weak an ensure justice, putting their lives on the line, those generally keep their powers even in adulthood. They're often called "battle magical-girls".
In either case, an active magical girl has two form. Their normal form, in which they are indistinguishable from an ordinary human, (unless you use X-rays to see their magical-core) and the one they take to use magic. Unlike in your world's TV show about magical girls, changing from one form to the other engulf a magical-girl in a blinding (and strangely, cold) light, preventing anyone from seeing their transformation, which not only replace whatever they were wearing by their ‘uniform’, fault of a better term, but also cause some physical changes. Often it is just a change of eye, hair or/and skin color, but it can be more drastic.
While magical-girls can be quite powerful, they aren't goddesses. Like any human, they grow old and eventually die.
…At least if they aren't killed in combat before.
Battle magical-girls are known for their endless fight against demons and other hellish entity trying to invade and conquer our world or kidnapping humans for diverse reasons, but they also have to face some evil organizations composed of humans. Most of those organizations make illegal research on how to take a magical-girl's magical-core and graft onto someone else, be it to have magical-girls that would be on their side, or some patriarchal sect convinced women are beneath them. The kind of macho thinking women should stay in their kitchen and obey like gentle little (sex)slaves, and as such try to create magical-boys.
Because while a boy's body also accumulate magic, it's far less efficient at doing so, to the point a man is generally far less adept at magic. And a boy having a magical-core? It's unheard off!