It is not going to be the same for everyone. Some of it is genetics. Some of it is how well people took care of themselves while they were in their prime. Some of it is accumulation of injuries (which of course is related to the previous two, also related to style of play and simply being lucky or not). Some of it is style of play which has a big effect independent of injuries. For example, the way, say, Hezhi Wen plays is not going to require the same athleticism as, say, JSH or Ma Long or FZD. Some of it is simply motivation. Some people at a point just want to do something else and don't train as hard or as well and may not even be aware of it. Think Zhang Yining for example. She quit while ranked #1 in the world and could have played longer. (Deng Yaping also if I recall correctly).
But you can kind of see that really good European players, who usually don't have to worry about losing their place on the national team, can be good into their 30s. But they can't play exactly the same was as they did at 22. Even some Chinese players could play pretty long, like WLQ and Ma Lin. I don't think they ever got hurt. But we see what is happening with ZJK.
I don't think there is just one year where there is a physical peak, rather a period of around 5-7 years of plateaued prime athleticism. Then things change, wherein some people may be a little slower on their feet and less elastic but may actually get stronger in other areas.
Think about this then. Harimoto is still just a kid. His "man strength" has not even kicked in very far yet. How good is he going to get when he is 25?