There are no level locked regions... unfortunately.
I don't think there should be level locked regions, but I do think there should be areas where you'll have to fight an extremely uphill battle (or avoid it entirely) if you're at lower levels. The whole "here be dragons," attitude.
Really ? That's weird, because in my original playthrough when I returned to places like Bleak Falls Barrow at level 53, which I'd entered at level 1 or 2, the loot was still all really low level.
They're level locked in the sense that once you enter them, they'll be at whatever level you were at when you first entered them.