6-7 maybe depends on how they tweek the XP gained killing lower level monsters. I don't like being level throttled on an act by act basis it feels like a waste of time killing things. If swen adds it I will instantly get a mod to remove it, it is a bad idea. Why would you stop gaining experience anyway? Because it is inconvenient for the plot for you to do so? Pfft whatever. lol.
Again monsters and characters placed along your path that conveniently get more powerful just because you do is stupid. It may work in other games to have level caps but in D&D there is nothing stopping you meeting a level 20 lich at level one because Mr Lich just happened to be there, you know, as the world existed before the player and not because of it? Narr it will be fine.
If you meet something at way too high level for you there is nothing saying you cannot come back later when you are right? Nothing saying you cannot "have a go" at killing that ancient silver dragon at level 5. Can't imagine anything being left of your party 1 second after as it would wet fart you into orbit but still.
Level 6-7 is a nice spot imo although I agree nothing in act one would pose a challenge at this level. I mean nothing at level 4 does so what's the diff?