Factual error: Ben Stiller buys Champagne in a drug store, yet in NY the only legal place to buy wine/champagne is a liquor store.
Factual error: The opening scene outside the pre-school is supposed to take place in Chicago, but actually is somewhere in New York State you can tell by the postcard-like registration sticker in the lower driver side windshield of the cars.
Factual error: Greg's flight back to Chicago was leaving LaGuardia at 2:35am, only problem is that LaGuardia doesn't fly past midnight, they shut down. They should've used JFK which is 24/7 and only 15mins away from LGA, or Macauthur which is even closer to Oyster Bay.
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.