Ace: "Which means there's no place in the new world for a rookie like me to cut in", Thatch: "Right...though, Red Hair did climb up from there and became an Emperor
and
-From WB's perpective beating a Shishibukai can increase your reputation as much as beating an emperor
(Rivaille's note: to be clear, WB said that if Ace wants fame he should just accept the WG's offer or that he should settle for a Warlord's head instead of coming for him he then said : "Why is he so keen on taking my head. If he's hunting the Four Emperors then Red Hair's head should be fine too. Could it be, that he thinks i'm easier to deal with because i'm old?)
Here WB rejects what he thinks Ace is thinking, namely that WB is the easiest to take out since he is oldest. He contrasts this with Shanks, the youngest Yonko., who he says is also a perfectly acceptable target.