This question applies only to the new Disney canon (The EU Empire continued to use clones and didn't retire them permanently until after Endor, so this question is irrelevant to Legends).
As far as I am concerned, clones can be differentiated between Clone Wars veterans and clones who completed training after the war. By the end of the war, veterans are equivalent to a 20-26 year old normal human depending on when they entered the war, and 50-64 by the Battle of Yavin. Such war veterans would have went past their prime before the events of Rebels, which is probably why those clones in particular are retired from active duty.
What I'm curious about are the clones who did not complete their training on time to join the Clone Wars. Throughout the war, there are plenty of cadets who are still years away from completing training (if Boba Fett can masquerade as one, then there are definitely cadets who still have 3-5 years of the 10-year training duration to go). The Republic Senate has also made at least one fresh order of millions of clones (given the quantity and the supposed expense involved, I doubt the Kaminoans have millions of combat-ready troopers sitting around waiting for the Republic to buy them, and are more likely waiting for payment to start fresh batches of fetuses instead). Either way, all of these new clones will only complete training after the war ends.
Whatever the case, what happened to these clones who have no war experience but nonetheless innately superior soldiers? There will already be cadets who will graduate in the year immediately after the war ends, and any freshly ordered troops in the final year will be equivalent to a 38 year old at the Battle of Yavin - almost at the end of effective fighting age in the real world, but that can differ depending on Imperial policy. That leads to the question: When were clones fully retired from active duty? Did the Empire stop new production but continue to use new graduates until all of them went past the age limit? That would mean that clones (the post-war ones, not the veterans) were fully phased out very recently before the events of Rebels and Yavin. Or did they phase them all out in the early years of the Empire? If this is the case, then what happened to all the paid-for clones who are still in training? Those are still in their prime. Were they just killed off or what?