Marvel will cast actors who are really close to the comic counterparts as that’s what they do. I also think the kids are critical to the FF so I’d cast the main crew a bit older.
Reed - should be an actor in his 40’s who seems really smart. Skinny. Like a guy who lost his virginity at 30 and drifts into his own mind. Ed Norton would be great. Adrien Brody. Jonathan Rhys Meyers would be a great dangerous Reed.
Sue - need to be a convincing Mom, so you’re talking a nearly 40 blonde. Again, smarts are important, but a sense of toughness even more so. I like Kathryn Winnick a lot so her or someone like her.
Ben - clearly just voice acting. Need to have a thick New York accent and sound like someone from a tough background. Honestly I think Vin Deisel is the best guy for the role.
Johnny - can be any 30-ish young buck actor. This is a Han Solo type role, it needs someone who can pull of comedy but who looks like every girl wants to sleep with them. I think someone like Zac Ephron would work.