I think I worked it out and I’m dead serious about this, this isn’t a joke. A character is at their peak when they are at their oldest age where they still have their original hair color.
Aging in One Piece works differently than in our world. In our world, our peak is somewhere between 25-30 years old. But I doubt this is true in the One Piece Universe. Kaido and Big Mom are said to be stronger than ever and they are 50-60 years old.
Characters can also live up to 140 years old or more.
Characters in One Piece also tend to keep getting stronger in a linear pattern unlike real world. However, Oda can’t really make a 100 years old human stronger than that same person at 30. So he uses hair decoloration to show that they have passed their prime.
Maybe it makes sense that Sengoku went white hair 2 years after his retirement.
He stopped training hard and started relaxing a lot more and thus got weaker/passed his prime.