Hancock joined Warlords 13 Years Ago (She was 18 Years Old)
Jinbe joined Warlords 11 Years Ago (He was 35 Years Old)
Doflamingo joined Warlords 10 Years Ago (He was 31 Years Old)
Kuma joined Warlords 2-4 Years Ago, after Ace Declined Invitation (He was in his Early 40s)
Also,
According to SBS & Viver Card, Crocodile joined Warlords in his Early 20s, that means it was right after Roger's Execution
And Moria also joined Warlords right after his Crew was Defeated by Kaidou, meaning He joined either 22 or 23 Years Ago
Also according to Viver Card, Mihawk was Super Famous even during Roger's Era, indicating He joined Warlords at Young Age, like Hancock
-------------------------------------------------------------------------
This is Very Interesting, because We now have Three Periods
Shichibukai System was clearly created after Roger's Execution & First Members were Crocodile, Moria & Mihawk
And tbh this makes Total Sense, as these Three stayed in Paradise & They were literally Introduced preventing Rookies from Advancing
And They were Successful, as No One managed to reach New World & Rival Rocks Trio (Anyone who tried received Krieg's Treatment)
However, 13 Years Ago, WG started Recruiting more (Hancock, Jinbe & Doffy), however each one of those Three were Recruited for Political Reasons (Doffy Blackmailed them, Jinbe only Joined to improve Relationships between Fishmen & WG + Protecting Sun Pirates, and Hancock also did it to Protect Kuja)
So technically, WG only needed Mihawk, Crocodile & Moria to help stop Golden Age of Piracy (Moria was considered Kaidou's Rival & He had Super Broken DF, Crocodile is a Very Powerful Logia & Mihawk is WSS, it's clear that these Three were considered Bigger Deal than those New Recruits). But then it started using this Warlord System for Political Reasons (Such as this Flashback, just look at how Kuma was Recruited).
Also these New Trio weren't in Paradise, They actually lived in Amazon Lily, Fishmen Island & New World, so They weren't really doing their Jobs as OG Trio, They simply were Recruited to not have them as Enemies, and all Three had Powerful Crews, while OG Trio didn't have (Actually Gorosei said that if They knew Crocodile had Baroque Works, He would have been Removed & his Bounty Increased, but They didn't mind Hancock being Queen of Kuja & Jinbe having Sun Pirates & Fishmen Support)
-------------------------------------------------------------------------
Now, let's go to 6 Years Ago,
Crocodile recruited Robin indicating that it was around that time He turned all his Focus on Arabasta Kingdom & nothing else
It was also around this time that Moria became Weaker due to relying on his Zombies too much & avoiding Combat for Years
(For Example, Moria never used an Actual Named Attack during entire Thriller Bark Arc, Oda was clearly hiding his Prime Power)
Why i'm mentioning this? It's cuz around this time, both Crocodile & Moria didn't try to Stop a Certain Pirate from becoming an Emperor, who is none other than Shanks, and same goes for Mihawk who clearly stated that He isn't interested in him after losing an Arm
(As for Hancock, Jinbe & Doffy, as i said, They weren't really doing their Job in Paradise, They can barely be considered Warlords)
So are We supposed to believe that Warlord System was always Successful, but when it came to Shanks, They didn't send anyone?
Ofc They did & that's how Shanks became an Emperor, He did what No One was able to in +15 Years, He Defeated a Warlord in New World
-------------------------------------------------------------------------
What People need to understand about Warlords is that as i said, They didn't need to be Seven, Crocodile, Moria & Mihawk were clearly Enough (No Rookie can Survive Mihawk's Swordsmanship, Crocodile's Logia & Thriller Bark's Trap), but the Problem is that once you increase how many Warlords there are, you can't go back, because if People Fear 3 Pirates & then you Add one more then that's great, but if you have let's say 5 Warlords & then They are reduced to 3, then obviously People will think this System isn't as Scary as it should be & Golden Age of Piracy will return
This is why once WG started using Warlord System for Political Benefits, They had to preserve that Number, They must find Replacements in case things go wrong, and that's indeed what happened 4 Years Ago, Ace was Invited to Join Shichibukai even though there was No Deal behind it, meaning it's not same case as Hancock, Jinbe & Doffy, it was clearly to Fill a Vaccant Spot. But Ace refused, and They eventually found a Replacement in Kuma
Meaning there was a 7th Warlord before Kuma/Ace who was probably Defeated by Shanks, and He either joined Post-Roger Execution or alongside Hancock/Jinbe/Doffy, but as i explained above, these New Trio didn't really Operate as OG Ones, if there was a Fourth who joined for Political Reasons then He wouldn't care about fighting Shanks either, so He must be another OG Member, a Pirate who was Famous & considered a Top Tier even 24 Years Ago (Just like Mihawk, Moria & Crocodile), meaning He is at least as Old as Mihawk
-------------------------------------------------------------------------
We don't know of any such Character who fit such Description from Characters We met so far, meaning it must be Someone much Older & from Roger's Era, and Strongest Ones at that time were mostly those who joined Rocks Pirates
However, it cannot be Ochoku or John or Gloriosa or Stussy or WB or Kaidou or BM or Wang Zhi
This only leaves us with Silver Axe & Shiki
Could it be that Shiki was Recruited to become a Warlord after He escaped Impel Down & had his own Territory in New World but got Defeated by Shanks? Same for Silver Axe, it's Very Possible tbh
Imagine having Mihawk, Prime Shiki, Prime Crocodile & Prime Moria as Warlords
Explains how No One reached New World until Shanks
(It should also be Noted that Shiki's Steering Wheel was gonna be used on Kuma's Design Originally, so maybe Oda always had Shiki as Part of Warlords, He also said that Shiki was gonna be a Villain in Manga, but then He decided to make a Movie about him instead)