What do you think is the best level to do the Dark Brotherhood and Thieves guild quests as the armor you get from them is levelled right?

Just started a stealthy archer so was wondering when i should do them for some good armor.

Most items stop leveling up at 45 but that seems pretty late to do the quests. If your on PC i suggest downloading a quest reward leveler if you really want to use those items.

DB has no levelled rewards, you might as well do them at level one.
In the thieves guild, rewards are levelled up to 46.

Your pickpocket, lockpicking and sneak are more important for these quests then your actual overall lv. Unlike some of the other quests, these are not full of combat. Thieve's guild frowns on you killing your marks.