1. Pacing. The problem with films with an aging cast (Paramount has faced this twice, once with the TOS cast, and once with the TNG cast) is they get tired and slow. There are ways you can work around that - to a degree - but just having your geriatric main cast sit around and talk more doesn't solve that problem for a sci-fi film. It has to move, and move quickly. Either recasting or writing a totally new set of characters/events to build and explore.
A film's pacing has nothing
to do with the age of the actors. That is a truly blatant misconception.