You can make that argument, although I'm sure the name value these stars had in 1982 had something to do with who was cast in the parts. And after all, it's not uncommon on Broadway to have stars playing characters much younger than the actors actual age. The actors in the film could certainly be believed in the roles in which they are cast.
However, what's more important in this play is that the characters are age appropriate to each other. In that sense, again, the actors in the film are fine. Alex and Monica (Robert Preston and Lynn Redgrave) seem to be a good pairing, age appropriate for each other. If Monica were a lot younger, it would change the nature of their relationship.
However, you're right in saying that the characters could have been younger, 15 or 20 years so. But the fact that they weren't younger doesn't impact the film negatively in my opinion.