Other mistake: When Ben Stiller and Robert De Niro went to the store there are head rests on the back seats, but on the way back from Owen Wilson's house they've gone.
Other mistake: Near the end of the movie, Greg says he's in row 8 on the plane. But when he's on the plane, we see there are only about 7 or 8 rows behind him. If the plane had only 15, 16 rows, the plane wouldn't have that middle section of seats.
Other mistake: When Greg asks the kid behind the counter if they have the Nicorette patches, he says no, and hands him a pack of the Nicorette gum. However, the package to the left of the gum, and possibly to the right, are patches.
Other mistake: When Ben Stiller borrows clothes from Robert DeNiro, he was wearing a beige turtle neck sweater. In the breakfast scene when Owen Wilson is introduced, Teri Polo wears the same sweater for most of the rest of the movie.
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.