The garden opened up 12 years before the start of FF8. I'm not sure if Quistis would be there or not, but I would definitely think that Squall and Seifer wouldn't be there (unless if they started at the age of something like 17).

Squall and Seifer joined sometime between the time they were 5 and 10, so its possible, but thats up to Jimi. As for Quistis, you were right, oopsies

Squalll and Seifer started, I believe, at around seven. I remember reading something that the group from the orphanage all left for Garden at around the same time, but Quistis could possibly be at Garden already. She was a SeeD at fifteen, so I'm assuming she started at Garden early. I dunno, I could be wrong though.

If the Garden started 12 years before the game, and were about almost at its first birthday, then the group would be around 5-7 about now. Squall and Seifer join around that time, but that depends on everyones opinion. But Quistis joined when she was 10, so that probably wont happen for a while. Right now i suspect she is either in the orphanage, with her foster family.