I am 37 and so far have not run into anything that was really all that challenging all the quests are super easy and all the "group content" is a zerg fest.

There is no level requirement for quests in this game as far as I know, if you want a challenge go do something above your level, I always quested 3 levels above my character level, but that's just how it turned out when I was running about leveling up. Also Cyrodiil bumps you up to what I keep hearing is "lvl 50 in lvl 48 green gear" so you can try your hand at clearing out the public dungeons there too, they all contain skyshards.

edit: forgot to mention, Cyrodiil unlocks at 10.