Think about the history of FF7 for a second.
1. Jenova discovered
2. Shinra makes soldiers by infusing Jenova cells into humans
3. Creation of Sephiroth as the pinacle of that experiment
4. War with Wutai- lots of Soldiers
5. War ends, Soldiers go home
6. Soldiers make babies.... because that's what they do after wars....
7. These children, some would be in their early teens, are born to those with Jenova's DNA hidden inside them, much as Sephiroth, but on a milder scale.