A majority of the film was shot in 1985, but to capitalize on a narration of the story, the scenes involving Sean Astin narrating the film were shot in 1987.
It was the directors idea to give some validity that he was telling the story at a later date.
Also, Alan appears to age during the 'flashback' as well there are scenes where he is visibly taller and has a lower voice. I'm not talking about the narration scenes, but scenes in the movie itself. There is a specific scene where Alan is trying to get a signal on his radio and is talking with Vic and he is taller and has a lower voice than most of the rest of the movie. There are also scenes where all 4 boys are standing in a line and are almost the same height, when earlier in the movie the two younger boys were inches shorter than the older boys.
My guess is the movie was cut together as the 1985 story and seemed very short and choppy so it was never released, then they went back and re-shot some extra scenes and shot the 'narration' segment in Central Park to pad out the movie and have the narrator explain some of the plot holes and storyline.