18- is still considered a child. Indeed, most people don't seem to be able to mature even after that, but that isn't the issue here.

The main cast are the 'Fated Children' because they're the children from the orphanage that reunited with eachother because of fate/destiny/all that great stuff. They're also SeeD, the children Cid/Edea adopted, etc etc. Nice soft symbolism doesn't need to be ruined by overthinking it.