[Bruce24]

There was some of both. Brown deaged was mostly in short, static scenes. Often, they just used regular Brown (with the kid showing up in mirrors). And sometimes they used the kid.

I suspect the kid wasn't much of an actor, because they didn't use her for many (maybe not any) of the speaking scenes.

What they did was kind of similar to how they did Luke Skywalker in The Mandalorian. They used the young actress as a stand-in who was then post processed via CGI into young Eleven. Her name is Martie Blair, and this is what she looked like at the Stanger Things premiere:

 

[Bruce24]

What they did was kind of similar to how they did Luke Skywalker in The Mandalorian. They used the young actress as a stand-in who was then post processed via CGI into young Eleven. Her name is Martie Blair, and this is what she looked like at the Stanger Things premiere:

After posting this I thought another comparison would be young Steve Rogers in Captain America, where they also used a smaller framed actor and CGI for the desired effect.

 

[Bruce24]

There were some scenes that were clearly just the kid (again, ones where she didn't speak). And many that were just adult Brown in place of the kid. I only noticed a few that were the CGI blend.

I'm not sure what your referring to, I believe anytime we see a face on the young Eleven they have CGI Brown's into the scene, if not it would be obvious.