You are able to start at level 51 if you wish to skips the normal, hard and challenging content. Regardless, there are checkpoints every 10 levels. Ex: Once you beat level 10 you can leave and come back starting at level 11.
While playing there are checkpoints every 5 levels. Ex: You wipe at level 17 you but will respawn at level 15. (At least that’s what I saw today.) However if you leave the game at level 17, when you come back you will start at level 10.