It's random, just like a lot of item finds in the game. I'll also guess your in the low to mid 50's because that's when Daedric items become more common.
I found a daedric dagger in a chest as random loot at level 25.
I love random finds like that. I mean really? Who places a weapon like that on an exposed tomb and doesnt expect someone to take it?
I always found a skeleton hand, holding a glass sword just west of Whiterun..Theres also always an orcish helmt in streams inside several caves.. I am the type who scours every nook, and cranny, and everfey bit of each map... I find everything... Books laying next to downed logs, chests inside trees, armor/weapons in streams, rivers, riverbeds, etc...
Guess thats why it took me 1500hrs and 8 characters to finally kill Alduin, lmao!
That item has always been Daedric for me, though It's varied in my playthroughs. Daedric Battleaxe of the Inferno, Lightning and Thunderbolts.
The lowest I've ever found a piece of Daedric is level 16, a Daedric Shield in the boss chest in the other fort just across from Fort Dunstad. (Can't place name right now)
The former is a great example of how far Bethesda has come with "Random but Contextual" loot. As opposed to the still overused Boss Chest. I'd rather see more use of the Dummy items to load loot in a more naturalistic way. It's one thing to pop open a chest and see some icons, it's an entirely different experience to find a cermonial burial site littered with the rarest of treasures. It was part of the reason I loved exploring in Morrowind, that "wow" moment when you stumble across a grand burial chamber littered with the most precious items.
The Daedric Greatsword of Annihilating has an invisible barrier that is three times it's length, so don't mount it near a door.