You're all a bunch of pessimists.
The reason why artists don't ever become great--in any field--is because they don't do the impossible, when what's "impossible" is actually possible. Luganski learned Rach 3 when he was eighteen, some others less.
Always challenge yourself. Who cares if her interpretation won't be as "great"? That doesn't make the piece untouchable. The earlier she learns it, the better she'll mature, the better she'll understand it when the time comes.
No successful pianist hasn't done something because of age. Age doesn't matter, maturity does, and facing a mature piece often brings that. Believe me. I know pianists who haven't even touched seventeen whose interpretations for certain pieces are unbelievable. Even Argerich performed mature Bach pieces as a kid. Larrocha performed music no one ever heard outside of her home country, and she remains the #1 interpreter of those works because she understood them and grew up with them since she was a little girl.
Was it just talent that brought them to their thrones? Natural selection? No. They challenged themselves, did what others said not to do and what others considered "immature" or impossible. And they won in the end.
It's too late for the girl--she's either learning it right now or isn't. But pianists shouldn't discourage other pianists unreasonably. The girl is obviously experienced and has played mature pieces, being a diploma pianist, learning Rach 3 can only do her good. She doesn't even have to perform it. She can just learn to understand it, hum the tune, play it for her own amusement.
She isn't even doing it for the sake of competition, she's doing it out of love since she truly is obsessed with the piece.
I swear, artists unreasonably discouraging other artists is a huge problem. I can understand if it were a beginner wanting to learn it, but a maturing girl who has experience? Please, get rid of the pomp, get rid of the discouragement, stop thinking an individual "shouldn't do something" because they aren't mature enough. Get rid of the fences that limit possibilities. Only then can you grow as a person and let others grow, too.