Wow, thanks! This one's completely new to me.
The movie premiered on Feb. 4th, 1991. Female lead
Karolina Lutczyn turned 15 on Dec. 11th, 1990 (only 7 weeks earlier). She couldn't have been 15 during filming. Probably 14, though 13 is possible depending on how far in advance they filmed.
As for time of year: I'm guessing the shoot took place in May and early June. Spring in Poland is a little chillier than in central Europe or most of the US with occasional warmer days thrown in. That's why in most of the outdoor shots people aren't dressed for midsummer. But you see the lush green foliage in the background - so it isn't fall. Another clue is the dandelions in the fields. They can turn from yellow flowers to white puffballs shedding seeds anytime during the warm weather but mass seeding events, when they all go at once, tend to happen in that time window I mentioned, around the end of spring.
Anyway, that's me using environmental cues again. Very helpful when a movie is filmed in temperate climates. Less helpful in the arctic or the tropics. If anyone can find an interview with the director maybe that will tell us whether production was in 1989 or 1990. I'll also look for some subtitles.