We'll you're talking about something like a 20 year difference between the two, right? Slattery can't really play 30-something Stark and Cooper certainly can't pull off 50-something Stark. As long as you have both of em to use, why not use em both? Having the founding members around doesn't mean they have to be at the founding in the scene. It could be a scene set as late as the early 70s even.
Atwell can be aged up more subtly and convincingly than Cooper could be, especially since an aged Cooper wouldn't look much like Slattery.
Atwell can be aged up more subtly and convincingly than Cooper could be, especially since an aged Cooper wouldn't look much like Slattery.