The movie could be called Young Avengers for branding purposes without the term being used in-story.
It could be a time travel story where the team finds themselves in the mid to late 19th century American West. Avengers: Young Guns
By Avengers 6, they might phase out Thor and Banner (unless they can get a movie for him)
I don't think it is Disney phasing out its current line-up of Avengers. This issue isn't even how old the actors are. I believe it is more about how long the actors want to keep playing their characters.