I see several discussions about the heyday of One Piece where they never reached a conclusion, I just come here to give my point of view. I believe the peak is around that age, because Garp vs. Don Chinjao, where Garp at 48 says, "Hey Chinjao! I'm going to break your head today, I trained breaking 8 mountains..." and then more some dialogues Don Chijao says: "I'm going to break that fist of yours that I'm so proud of, Garp!" Considering this information, I interpreted it as follows, previously I could not do such a feat and also Garp never did a haki collision between his fist and Don Chinjao's head, thus having to get stronger, where a person who is not in its heyday would be unable to do such a feat. Roger at 51 years old did not complain about exhaustion, old age or his illness, leaving him to the point of spitting blood and making him fall to the ground, even so, he was able to fight 3 days with Newgate with all his strength, maybe, here begins the stagnating the strength of the character, where he does not lose his strength, but is also unable to increase it or is still at its peak(?). Big Mom at 68 years old also never complained about any of the above conditions, thus leading to believe that old age in One Piece starts at 70 years old, as Akainu also says that Newgate's strength was not the same because of his old age.