Thaddeus Winslow Cooper III was considered the most chivalrous Cooper of them all. He, like most other Coopers, left his cane and other belongings in the Cooper Vault. It can be predicted he may have been more of a civilized individual than a thug; he possibly stole in a more 'polite' pickpocketing way or used his disguise skills to carry out heists. Thaddeus' section of the Cooper Vault consisted of acrobatic feats, such as swinging from cane to cane, etc. Despite looking rather old compared to the rest of the Cooper Clan, it can be assumed that he was an incredible athlete. Thaddeus was at the climax of his thieving career during the 1890s described from his Thievius Raccoonus entry about disguises.
His most recent appearance (or, rather, mentioning) was during a heist in Bavaria, in which Sly used his section of the Thievius Raccoonus to master a more gentlemanly disguise to fool Carmelita and steal a key necessary for the heist. Jokingly enough, Sly's disguise made him look very much like Thaddeus.
If you look closely at Thaddeus's page in the cartoon short "Goodbye My Sweet" it has two separate years as to when it was written 1839 underneath the title and 1893 underneath his picture.
Thaddeus at one point was employed by the Scotland Yard, as his entry in the Thievius Raccoonus states.
Due to the small gap between which their carreers reached their zenith, it is possible that Thaddeus and "Tennessee Kid" Cooper are directly related (Tennessee was active in the 1880s, only a decade before Thaddeus reached his climax in the 1890s)
In the Inner Sanctum in Sly 3: Honor Among Thieves, when Sly talks to Dr.M, he says "Thadius", instead of "Thaddeus".