is there a group sync for quests and leveling in the game, like in WoW ? I would like to play with a friend, but is it possible to replay content that I have already done with him if he is lower level than me ?
Don't worry it shouldn't impact his experience if hes low leveled. Everything is scaled, he may have alittle more trouble in the dlc zones because they are supposed to be harder than normal zones however I don't think it will impact gameplay much, just alittle more troublesome to kill mobs.
Okay, so assuming my friend is level 15 and I am level 30, we can still play together ? (I talk about normal zones, not DLCs ones). The rewards and the ennemies will scale for both of us differently, right ?
In ESO enemies don't scale, instead players below level 50 are scaled to an equivalent power level of a level 50 at CP160.
As a side effect, you will see a considerable performance drop when you reach level 50 (and Champion point level 10) since as long as they keep their gear up to date, your lower level friends will still be bolstered up to CP160. (Luckily, the trudge from CP10 to CP160 needs to be done only once, your CP level is server wide, for all chars.)
At level 15 and 30 you both are mainly limited by the skillpoints you have, so there's really nothing stopping your from questing together. DLC zones are fine as well, the only zone you should avoid before CP160 is Craglorn.
You also can just join in when other players are fighting a world boss, for example. No need for grouping up, everyone will get credit for the kill and the 12 players with the highest dps will get loot scaled to their level. Nobody loses anything by helping each other out.
Okay, so assuming my friend is level 15 and I am level 30, we can still play together ? (I talk about normal zones, not DLCs ones). The rewards and the ennemies will scale for both of us differently, right ?
The only hickup would be that you can't trade gear you find if the other player needed it. (Your's would be level 30, and his would be 15.)
You should end up pretty close to each other in levels by the mid 40s, simply because of how XP to next level scales. Once you're both at 50/160+, it won't matter because you're both at gear cap.
It depends on what you mean by "replay content." If it's a quest, I don't think you'll be able to repeat quest-related content that you've already done, so you might not be able to play alongside your friend if the content is instanced according to the phase of the quest that's in effect. And certain quests are solo instances, so you wouldn't be able to do them together even if both of you were doing them at the same time.