Lyse's age is not directly disclosed in the game, but it's assumed that at the events of A Realm Reborn, she was 25 years old. In Final Fantasy 1.0, Ala Mhigo had already succumbed to Garlean by 15 years. A Realm Reborn fast-forwarded five years in the future, which gives us 20 years since the fall of Ala Mhigo. Since Lyse was five years old when all this happened, the bill fits.