Okay, theres not going to be two actors playing Doom. Im pretty sure of that. Yes, hes the Darth Vader of the Marvel universe, because he helped inspire Darth Vader, but that doesnt mean hell be portrayed as Vader has.
Im not totally against the idea, but Im sure its not the route theyll take. Regarding age, I think early 30s good. Remember, this would be the start of a new series within the franchise. Im thinking Reed Richards is cast less like RDJ in the first Iron Man and more like Pratt in Guardians. Someone with a sensible enough amount of exposure. Someone younger than those weve championed for the role prior (Krasinski and Isaac.)
Doom should be within that same age range. I also just really like the concept of treating Doom a bit like Thanos, teasing him in the background, just not as drawn out as the former was. If one element of Noah Hawleys treatment makes it into the MCU, Id want it to be Doom as a sort of anti-Iron Man/Doctor Strange. Someone who goes through what could be considered a heros origin story but ultimately comes out as a villain. The villain.