In Love with an Older Woman (1982)

Robert (John Ritter) is a lawyer who takes more than professional interest in the new investigator hired by his firm, Isobel (Karen Carlson). As the two start working together, a romantic relationship gradually develops between them. But there's one slight problem: Isobel is 43 years old, while Robert is 29. The difference in age increasingly becomes a barrier to their romance -- especially when Robert discovers that Isobel has a 24-year-old daughter.