Appropriate Range

If the party finds a dungeon and asks “Am I the appropriate level for this dungeon?” the answer is always “yes!” For the terribly meta-gamey reason that you’ll almost never encounter content in a tabletop RPG that you’re not meant to attempt. Unlike video games, where the world just exists, it is rare for games to be such open sandboxes that the players will encounter content that is far outside their level. The DM is highly unlikely to have spent the time necessary to create a dungeon for six levels in the future.

Really, in many ways, levels are irrelevant in a lot of modern design, as the party will typically fight level-appropriate challenges: they’re static in effectiveness. The difference in a level 1 fight and a level 15 fight is one of background flavour. I think this is changing with 5th Edition.