From what I understand it’s anyone with over 15 percent of something in polls and something about being eligible to run for enough electoral votes. I don’t really know what that last part meant other than perhaps they have to be eligible to be on enough state ballots for the required electoral college votes.
Many swifties are well above that age I’m in my early twenties and she was becoming popular when I was in elementary school.
Edit: She also released her first album the year I entered school.