The lvl 1-50 "phase" each zone is basically a fixed level range , much like you would find in WoW (i.e. all quests in a given zone is doable if you are in certain level range). It will not scale to you, so if you skip too many things you will eventually run into a zone being "under-leveled" and start to struggle. So there are no restriction in picking up the quest, but another story whether you can complete it...
I know Zenimax made some sort of effort to make the game more "immersive", so they hide some of the "technical" detail . I think the quests have a color range instead of a lvl range, i.e. yellow/red/white/grey to indicate whether you are in its range, but mostly you have to go on the mob levels off the zone itself.
Now that said, once you hit lvl 50 and/or finish the main story campaign, you can "replay" the other 2 factions' campaign, THEN the zones will scale to lvl 50. So if you rolled say, Ebonheart Pact , you will play a standard Ebon campaign from lvl 1-50 with zones being lowbie level ranges. After that, you can then play say , the Daggerfall Covenant campaign , but then the starter zone will suddenly be lvl 50, and next zones will be VR rank 1 , 2 ,3 etc etc. If you feel you rather want to experience the Daggerfall campaign as a lowbie, then you need to roll a new character etc etc.
Anyway, as for the campaigns itself, i still feel, the Daggerfall covenant has the best starting experience (the desert oasis zone). For some reason it has quest types that i did not find in the other campaigns' starter zones (like that riddle-treasure-hunt quest line ).