No idea if that was societal norm, mens fantasy, or whatever. I have to kind of believe movie execs would want the largest audience and so if 40s-50s men dating 20s women was not appealing to women of the time they'd been losing 1/2 the market. But, maybe these particular movies were the male equivalent of romance novels which seem to be written 99% for a female audience
Some examples:
The Girl Can't Help It (1956)
South Pacific (1958)
My Fair Lady (1964)
The Far Country (1954)
Casablanca (1942)
To Have and Have Not (1944)
Charade (1963)
There are literally 100s of them and I'm not recommending any of the movies above per-say, but it does stick out to me just how common that theme is of man, at least 15yrs older than woman which I believe is less common today