TL; DR = My campaign stops level up at 5, or 8.
In my previous campaign, 4e, we took the characters from 1st to 28th (aiming for 30th, but they didn't quite make it by the final battle). Whatever else you might say about 4e, I found it was pretty balanced at all levels of play, between PCs and PvE. (Although we did "hurry up" some encounters by halving monster HP and doubling their damage.)
In my current campaign (5e)... I hard-stopped progression for the world at 5th level. And in answer to a question on the first page of this thread, that means humanoids - with few rare/legendary exceptions - cannot advance... but the dangers in the world are NOT so limited. The starting map for the campaign that was handed to the first level characters included the rough location of an ancient red dragon! This limit applies to "good" and "evil" races alike, with most "normal" people being 2nd or 3rd level (maybe in sidekick classes, not PC classes), while exceptional people hit 4th or 5th.
Certain story triggers were predefined to allow the PCs to "break the mold". Some have been triggered, some have been found and declined (they came with side effects), and some haven't been found; the PCs are all 8th level right now, except one who was willing to risk the side effects and is now 9th level.
(Having said that, when characters would otherwise "level up", I have rewarded them with bonus feats, or additional powers. Likewise, many NPCs the party encounters have the benefit of an extra "feat" to reflect experience and training. The Royal Guard might be only 4th level, but they have group tactics, battlemaster maneuvers, and multiple Fighting Styles!)
Some NPCs in the world "get around" the limitation by multiclassing. For example, one foe they faced was the Swamp Witch, a Sorcerer 5 / Druid 5. An important ally/rival of one PC is a Bard 4 / Rogue 3. (Both NPCs have, for Story reasons, also qualified for one of those aforementioned mold-breaking triggers.)
I imagine the PCs will probably finish the campaign - sandbox, so "finish" is "let's move on" - around 10th level maybe 11th... with lots of bonus feats and powers, and a handful of side effects from the crazy and warping things they've dealt with. But at the current level of play the martials are crushing single-target damage, the casters provide "outside the box" tools and solutions, and low-ish HP keeps everything dangerous. It seems to be the perfect mix. (And I don't have to worry about teleport or forcecage or or or or!)