Originally Posted by
Vivi22
Aeris is only 22, so it's unlikely it happened 20 years in the past, particularly when the incident with Sephiroth and Nibelheim was shortly after the end of the war. At most it might have been ten years earlier, which could put Biggs and Jessie in their mid-late teens at the very least if they were in their mid twenties when the game starts (not exactly a bad presumption since Cloud is 21, and we can assume Jessie is at least the same age.
That said, the game seems pretty inconsistent to me as far as the timeline of characters before, during, and after the war.