18- is still considered a child.
I dont think so.

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.
But when they defeat Ultimecia they are not kids anymore.
The are matute.