Depends upon your strategy. Usually levels in the high teens is a good idea for the Ice Cave, but I know a lot of people that do the cave by just running from every random encounter. If you follow that approach then you only need to be in the low teens to get through. Either way, make sure you save before entering(although you should be in the habit of saving before any cave now) as the enemies in the Ice Cave are some of the most frustrating in the game.