As the show’s fifth season got underway, many fans wondered if something had happened to the actress playing young Kate. There were some who believed she had been recast because she looked so different from the young Kate that they were familiar with. The explanation is actually much simpler — Mackenzie Hancsicsak has grown up, and the show is having more trouble hiding that fact.
Mackenzie plays Kate at age 13 and is that same age in real life. As most parents are well aware, 13 is the age when many kids go through pretty radical changes in appearance, which can lead to some confusion.
“Does the girl who plays young Kate look like an entirely different actress or is it just me?” one person asked on Twitter.
“Hold on, is that a different young Kate up here??” another added.
Although Mackenzie and the rest of the actors who play the younger versions of the show’s characters have definitely grown up, there’s been no indication that the show plans to replace any of them. Instead, it seems This Is Us is planning to depict different, more mature flashbacks in its episodes, or simply ignore the fact that its young cast looks different than they did in earlier seasons.
Source : https://www.distractify.com/p/what-happened-to-young-kate-in-this-is-us