Ancients are a colloquial term used to refer exceptionally long-lived individuals. Since it is a colloquial term, there is not a universal definition, but a good rule of thumb in determining whether or not someone is an Ancient is whether or not they were born before 2100. As of modern day in To the Stars this puts all Ancients over the age of 350.
Much of the additional abilities Ancients have over most other magical girls and people in general typically comes down to experience. One of them is their increased social prowess and emotional control. The story comments about these abilities in passing, usually during narration.
Clarisse moved through the rest of the team, shaking hands and smiling charmingly. It was almost unnatural for a girl who looked so young to exude so much charisma, but at this point Ryouko was beyond being surprised by that kind of thing. She was sure that Clarisse was at the point now where she could turn it on and off like a switch, similar to the other Ancients she had met.
Another notable ability of Ancients is the great versatility in their magical abilities. Some of this can be attributed to the large amount of time they have been alive and could develop through the process of power development, developing powers related to their current skill set. But perhaps more interestingly, Ancients also are able to pick up powers in completely unrelated areas, called "Ancient Power". The mechanisms of this are not completely understood and many theories circulate about it. Clarisse van Rossum seems to believe though that:
"Ancient Power" is somehow a response to a persistent need—a magical girl who constantly needs to move quickly develops teleportation, for instance.
Since all Ancients are necessarily magical girls, Ancient culture is deeply influenced by the experiences of Magical Girls in a resource-scarce past. Below are a few examples of this in the story.
“Sometimes I miss the old days,” Kyouko said. “Just the four of us, alone in the world, fighting demons. None of this complication with the MSY, and the government, and the military, and the aliens.”
Mami looked up too, allowing herself to be drawn into the reminiscence.“I understand,” she said. “Even though it hurt being so alone, thinking back, we really had something special.”
Watching about Homura’s childhood, she pondered just how different a world the “Ancients” hailed from. No one to rely on but you and your team, always living on the edge of survival, a world where failure didn’t mean someone would pick you up so you could try again, but death, plain and simple death. A world where you couldn’t even leave your section of the tiny city without risking conflict; a world where you always had enemies, whether you wanted them or not.
Oldest Living Magical Girl: