Harrison Ford's age actually match's up much better with Han's if they go with 40 years later, actually.
Han is supposed to be 33 in RotJ. Harrison Ford was 39 when they started filming. He'll be 72 in July and they start shooting this in May. If Han is 74, that's much closer to his actual age.
Mark's and Luke's ages actually match up perfectly, as Luke was 23 and Mark is now 62.
Carrie would be playing a Leia who's several years older than she is, due to the five year gap between her and Mark.
Lando and Billy Dee are both 76.
Wedge and Denis are both 66.
It makes way more sense to set this 40 years later.