My mistake! The only previous production of Sunset Boulevard I've seen was the 2017 revival, where the ages of Glenn Close and Fred Johanson were so obviously mismatched. I guess it never really registered to me in the film that Erich von Stroheim was considerably older than Gloria Swanson, I just thought Norma had put more care into preserving her appearance. But all you and AlanScott say makes a lot of sense. Anyway, Jamie Lloyd's expressionistic productions forgo the logical necessities of realism, so it didn't bother me at all that they were of a similar age.