What level is recommended before starting Nightmare/Hell/Inferno?

In Diablo 2 it was very possible to speed run through the acts and end up on a harder difficulty with a lower than anticipated level.

This is quite achievable in Diablo 3 as well, whether bypassing the optional content or being rushed by a friend.

What are the recommended level ranges before progressing to the next difficulty?

Normal Mode should take you from levels 1-30.
Nightmare Mode should bring you from 30-50.
Hell Mode is tuned for level 50 characters and should take you to the level cap of 60.
Inferno Mode is designed for level capped players. You can not access Inferno mode without reaching level 60.

Normal is designed with the assumption that you will be level 30 by the end of it. Playing through Nightmare should take you up to level 50 by Hell, and you should leave Hell at the max level of 60 ready to take on Inferno. (Or not!)

Normal 1-30

Nightmare 31-50

Hell 51-60

Inferno 60

normal: 1-40 depending on how much you explore, master level, and any bonus exp from gear, and bonus exp for playing with more players. The average seams to be around 33-35.

NM: 25-50 the minimum for Nightmare is 25 and hell is unlocked at 50 so technically it's 25-50, but most start low 30s and end at the 47-52 range.

hell 50-60 agian that's just the level restriction, but some players start earning paragon levels in hell mode so it's more like 50-P3

Inferno Lv 60-P100 once you reach level 60 you gain paragon exp until you max out at paragon level 100.