I thought everyone in Coco becomes middle-aged when they enter the other world but in the last scene when Mama Coco dies she looks older than her parents..
Director Lee Unkrich answered this question in a tweet. The age a person appears in the afterlife is the age they died.
@PogieJoe: Hey @leeunkrich in the Land of the Dead do the characters appear as the age they died at, the age they are in their photos, or some third option?
@leeunkrich: Age they passed away.
Disregarding whether it makes sense or not, everyone on the movie seems to have the same aspect they had on their offering photos (or A photo for those who don’t have an offering).
Out of universe the reason is obvious: so you can recognize them. In universe is not established so we may connect those two facts however we wish.