Theory on the creation of the Shichibukai.
The Shichibukai's (main) purpose is to maintain a the balance of power between the Yonkou and the Word Government. Given that Shanks rise to power occurs somewhere over 11 years ago and Kaidou seems to have risen to power around the time he defeated Moria over 10 years ago. It is reasonable to assume that the Shichibukai was instated around that time.
Approximate Shichibukai Member Join Dates:
Hancock: Roughly 11 years ago. Possibly 10, depending how soon her first campaign after being declared empress was and how quickly she was approached with the offer.
Mihawk: Unknown. Though Shanks lost his arm about 10 years ago if that means anything.
Moria: 10 years prior found Dr. Hogback, started zombie army. Possibly joined shortly afterwards but not before.
Jimbei: Nami spent 8 years making maps and stealing treasure. It does take time to go from Fishman Island to Nami's Island. So Jimbei receiving his title 9 years ago isn't a stretch (neither is 10 depending how long it took Arlong to settle down.)
Kuma: Unknown, if he is a spy for Dragon I don't see why he wouldn't have joined as soon as possible, though Pacifista development seems to have been quite recent.
Crocodile: I'd say over 4 years ago, how many more is anyone's guess (when he approached Robin), possibly longer.
Domflamingo: By deduction he is probably the newest member joining at earliest three years prior (Ace was invited at earliest three years ago, probably closer to two.)
Blackbeard: New, replaces Crocodile.
If this is the case Hancock, Mihawk, Moria, Jimbei and possibly Kuma seem to be original members. Crocodile followed (thought we don't know when, he is possibly an original member) and Doflamingo seems to be a recent addition.
Some assumptions that lend credence to the theory: the reputation of the Shichibukai would not be as great if there was a constant rotation of membership. This theory proposes that the Shichibukai has largely remained intact since its conception. Although, these pirates are already infamous with or without their title so this might be a moot point.