Quote Originally Posted by Vivi22 View Post
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.
We don't know how long the war went on for, though... Aerith sure seems to be an infant in that flashback sequence, though with FFVII's graphics it's nigh impossible to tell.
Still, we can assume that the Wutai war ended some time before Cloud left Nibelheim to join SOLDIER, since Cloud knows Sephiroth as a famous war hero but he never shows any fears about heading off to war.