I don’t think the cast will be too young - I think we (i) have aged up.
I read these books in 1995 when Robert Redford was still hot & we were all still swooning over Tom Cruise in a Few Good Men & in deep mourning for River Phoenix.
RCB is 37 when he marries 19yr old Taggie. I mean, Rupert was quite old (to 15yr old me) back then. He’s 60 in Mount! (2016) so lots of the people we are casting (in our heads) are the right sort of age for the characters now… but not when the book is set.
I agree that a darling buds of may C Zeta Jones is a perfect Taggie - but she’s 53 now - too old to be Taggie today! I can’t get my head round the casting of RCB or Tony though.
I hope they’ve got great people for Archie, Grace, Mrs Makepiece, Joyce etc.