Dungeons have a maximum/minimum set level. I,e Dungeon A is level 15-20. You go in at level 17 it locks to 17. Dungeon B is level 45-50. You go in at level 1 it locks to level 45.
It's basically a reduced form of level scaling, except the dungeon never levels up.
I would suggest that they do level up, after you've surpassed their max level.
Example
You go into to dungeon A, a level 10-20 dungeon, at level 6 and can't complete it. Dungeon is level 10 for you now and will remain locked level 10 until you're level 21 at which point it will unlock and become level 20 the next time you enter.
This retains the feeling of reward when you enter that dungeon after gaining some levels and can beat it, but doesn't make some players worry that dungeon exploring early on will lock dungeons at their lowest level and they won't later be able to get as much loot or challenge out of them.
Thoughts?