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No, it matters not. The only instances you should be aware of your level are those of which you expect to get certain gear. Then wait until CP160 if that matters to you.
Ever since One Tamriel, the level of your character doesn't matter anymore when it comes to accessing content. Everything is scaled to you. You can do the Dark Brotherhood as a Level 50 just the same as a Level 5.
The only thing that matters is timeline continuity. The Thieve's Guild and Dark Brotherhood take place after the events of the Main Story Quest and Wrothgar.
TG and DB can be done at any level, story wise.
Technically anyhow.
However... some of their questings and dailies do take you all over the basic maps, so it is less stressful if you do them after doing cadwells, that way you will have everything unlocked and explored.
But you can -start- them whenever, and just let them lay there once one of those "away missions" pops up...
Also, as has been mentioned, the item sets there are a bit on the rarer side, so it does make sense to wait until CP 160 to go, so they can be endgame-useful (and guildstore-sellable if they are not to your liking)
Otherwise, Thieves Guild can be started anytime. Some side quests in the Dark Brotherhood involve characters from all three faction storylines, but the main quest does not. Personally, I enjoyed doing Thieves Guild and Dark Brotherhood together, as the Blade of Woe can be handy in TG missions if you are okay with your thief murdering people.