A players "peak" is determined as the point in his career where he is at his best
The way I see it, there are two types of peaks.
Natural peaks and
actual peaks. A natural peak is how a player
would have peaked if he was allowed a natural progression, consistent playtime, and stays away from serious injuries. An actual peak is how a player
turned out to peak. To make an extreme example: if Januzaj was unable to play football for the rest of his life starting now, he would have peaked at age 19.
The problem I have with the Kagawa critics, is that it sounds like they genuinely think that Kagawa's natural peak was during his last months in Dortmund. As if the ball was just magically rolling his way the whole time, and that he would never be able to reach that height again, no matter the circumstances. They choose to have this belief, because they want to put all blame exclusively on the player.
I'm not suggesting that all players would have their natural peak at age 28. Some players peak earlier, obviously. The thing is, Rooney is said to have peaked at age 26. And that is considered an especially early peak. And now we're talking about a player who was a PL pro at age 16(?) and has played a shitload of games at top level ever since, half the time with a team built around him. Of course he's gonna peak earlier than normal.
But Kagawa naturally peaking at age 23, after only 2 seasons at top level, is just a bullshit theory. He may not reach his Dortmund form again, but that is because he went to
us. Had he stayed in Dortmund or gone to a team that suits him more, then he would most likely have stayed on the same level(or improved). It's preposterous to suggest that he would have ended up with his Moyes-form regardless of circumstances.