Trivia: In the scene where Greg is entering the drug store, listen to the lyrics of the music playing on the speakers - they are very similar to his speech when he says grace in the dinner table scene. Obviously where he got his words from, but not immediately obvious unless you listen for it.
Trivia: This film is actually a remake. The original "Meet the Parents" was an obscure independent film released in 1992, and was noticeably a much darker comedy. The rights to the film were eventually purchased by Universal. While this film became a massive hit, the original film has remained very obscure and is very hard to track down.
Trivia: Originally, the film was meant to be a vehicle for Jim Carrey. Among his contributions was Greg's surname, "Focker." Eventually, Carrey left the production, and Ben Stiller took the part.
Trivia: The scene with the lie detector came about as a suggestion from Robert De Niro after he learned about them from a friend he had in the CIA.
Answer: Who says they had to have been legal adults to date when Top Gun was popular? They could very well have been teenagers and got engaged shortly afterwards. If Teri Polo and Owen Wilson are playing characters that were born the same years as the actors themselves they would have been 17 and 18 respectively when Top Gun was released. If they are playing characters a few years older than they actually are, which is entirely plausible, what Pam says makes perfect sense.
BaconIsMyBFF
I would like to add that a movie doesn't stop being "very popular" soon after the release. In 2000, when I was in middle/junior high school, we actively talked about movies that had been released three or more years before (Forrest Gump, Scream, Cruel Intentions, etc.). A movie from 1986 could easily be popular among a dating/engaged couple and their friends in, say, 1990.