I´ve been watching another time the part where Teach fights against Ace, when he says that thanks to the Yami Yami no Mi he won´t lose against against guys like Ace, who are users of powerful devil fruits. We know that Ace is an user of an elemental logia and he is very strong going toe to toe against Jimbei.
But watching Impel Down saga, just where Crocodile says "thousands of silver medalists who lost against Roger or Whitebeard and are waiting their opportunity"
And the scene where Shirohige easily defeats Ace is shown, in this scene Ace isn´t a Shirohige Pirate yet, he could be one of the silver medalists that Crocodile talks about. In the case of Teach he could have received the same treatment than Ace from Shirohige.
What is the secret under Shirohige power?
From my point of view he has the Time-Time no Mi, a fruit that gives the user the hability of controling the time, this isn´t like Noro Noro no Mi from Foxy, I am talking about a devil fruit that can be activated all the time and gives the user the possibility of time manipulation, this includes time stopping, decreasing the speed of time or increasing it at will of the the user.
The first time that we see Shirohige we know that he has an illness, later when Shanks has his meeting with him Shirohige talks about the wound that Roger did to him and we know that Roger received a fatal illness.
is it possible that Shirohige received the same illness?
Roger lived only 4 years, Shirohige after 23 years continues being alive, people believes that is because the health assistance that he is receiving, I believe that is from his use of the Time Time no Mi, this devil fruit gives him the opportunity of living more and more.
When Shirohige talks about Sengoku and Garp he seems to be talking about people from distant past and we know that Garp and Sengoku are alive and kicking.
You can see it more reflected in the anime, I don´t know if it is an error from Toei Animation, it could be, but Shirohige remembering the days against Roger, Garp and Sengoku as distant past can be because he is giving himself more life thanks to his devil fruit.
The other proof is that we have seen how the people around the age of Roger have got a more older aspect than Shirohige who seems to be a more younger than them even when they are in the same generation. Is this an error from Oda or Newgate seems to be more younger than Rayleigh, Crocus, Garp and others?
One day I read that Oda met a man in a bar, this man was fightning against a mortal illness, he was fightning against time and at the end this man died but it was the reference that Oda used for Whitebeard, in him we can see the subject of a mortal man winning time against an illness.