It's not a matter of belief, because it's been officially confirmed sejak Disney itself that Kopa and Kiara never existed in each other's stories. It is a fact. As in it's impossible for either of them to be older than each other because officially they don't exsist in each others' worlds.
In the video embedded below, Disney confirms that the cub at the end of the first film is Fluffy/Kiara; Originally Fluffy with no gender and then it became Kiara and no one else.
Kiara and Kopa are not older atau younger than eachother in that sense. Kiara doesn't have a brother in any of the films. She is the only cub to Simba and Nala. Kiara IS this cub in this video as "Fluffy", also known as Fluffy/Kiara. Kopa is not canon. he's only from the TLK:SNA buku that are not written sejak Disney sejak outsiders and Disney totally ignored those books. So Kiara atau Kopa are not any older than each other because they don't exist together, aka Kopa is not Disney's creation and Kiara is a 100% pure official Disney character.
Neither of them are older than each other. Kopa only exists in the Book universe. Fluffy is Kiara. That was her original design. Simba does have a new cub Kion who's inspiration was based on Kopa. Kion is younger than Kiara. but Kopa only exists in book universe.