1: There are 12 greater clans/ council of 13 who are they and for what clans?
So far the Clans confirmed to be on the Council of 13 are the following:
Clan Moulder - Beastmakers
Clan Eshin - Assassins
Clan Pestilens - Plague Rats
Clan Mors - Warlord Clan
Clan Rictus - Warlord Clan
Clan Skurvy - Sailors and Shipmakers
Clan Skab - Warlord Clan
Clan Skryre - Engineers
2: Is there an exact age of these lords? In the book I believe it says none have changed since it's creation "except for clan pest"?
The Lords of Decay have been around ever since the Lords of Decay were first formed, some were older and were Grey Lords before becoming Lords of Decay. So they are around 200+ years of age.
3: Lords of decay vs council of 13? Are they the same thing or is the lords of decay a clan pest and their lackies thing?
They are the exact same thing. The Council of 13 is comprised of the Lords of Decay.
4: When ever I heard of the council they apperantly are more evil or something along those lines. Are they suppose to be bigger? or anything notable other then just a aura of uhoh?
They are more powerful than the average Skaven yes but mostly they are smarter and more cunning than the rest, which Skaven consider to be real strength.
5: I never heard mention of when clan eshin got onto the council did they ever get on it?
Clan Eshin have been on the Council ever since the War with Clan Pestilens, they were granted a Council seat for being the decisive factor that ended the war in the Council's favour. This was during the days of the Grey Lords though.
Hope that helps, i'm not sure about question six.