Twilight is an alicorn, now, and that makes her special. While Princess Celestia has made it clear that Twilight's new stature will not grant her immortality, and that she will age just like any other pony, being such a highly ranking magician gives her special perks, like a seat on the Council of Immortals.
Sounds like a good deal; every month she gets to learn new things from the oldest and most powerful beings in Equestria, but there is one, small…responsibility.
She's become the legal ward of a hot-headed immortal with a death wish.