I'm trying to find out at which level Elven Items start appearing, I searched the UESP but it didn't say anything.

15

http://www.uesp.net/wiki/Oblivion:Weapons
http://www.uesp.net/wiki/Oblivion:Armor

UESP does say when Elven stuff appears. It's level 9 in loot and level 13 in shops for weapons, level 15 for armour to appear as loot. You can't get Elven armour in shops, only in loot or on human enemies. You can find elven shields in loot as early as level 10 though.

But remember that when you level up higher the elven gear pretty much goes away and is replaced by daedric etc.

I don't think I'll ever get over the fact that someone thought this level-scaling system was a good idea.