It depends on the level you are when you finish Krokotopia mainline quests. There are no level requirements for first arc worlds and you don't gain access to a world just by being a certain level. Saffron Sandsword Legendary
To get to MarlyBone, you'll need to defeat Krokopatra, a Lady Krok, and she's in the Very Last Dungeon and Area, The Tomb of Storms- Wolf DeathWraith, magus