Well, true, she didn't appear to have aged, at least not in the same way as her friends as they all had the wrinkles under their eyes. Hell, Rarity even had that streak of gray in her hair, but that doesn't necessarily mean Twilight is immortal. Perhaps being the ruler of Equestria simply results in a magical size increase rather than natural aging. The possibility remains that once she relinquishes said title, she will age again as your standard pony would.