Mrs Robinson?
No, this was about a 15-year-old who had an affair with a 23-year-old married woman during WW2 - while her husband was fighting the war.
From the movie: ‘Summer of ‘42’, Starring Jennifer O’Neil. A very well made movie that captured that early WW2 year, and a 15 year old young man’s first love, (With a gorgeous older woman to boot), extremely well.