No, you only ever compete against skaters in your own level. (Unless they combine levels, but that is generally with the next closest level- such as adult pre-bronze and bronze combined into one event.)
What would happen is rather than a giant "preliminary" event, there would be 3 different events. Say "under 7", "7-12" and "12 and up" (i'm making those ages up- I don't know what age is typical for preliminary". So you would compete in 12 and up.
So other skaters in the lower level, but older.
But if the competition did "16 and up" you very well could be the only skater. That's when you skate "against the book"- meaning you are judged at a set standard for the level and need to gain 90% of the points to win first place. (Although I read that only ISI competes against the book now, and USFS automatically wins 1st... let me see if I can find anything about that.)
ETA: I can't find a USFS rule, but did find a thread in our archives about it. Apparently in USFS competitions if you are the only one in your group it is treated as an exhibition, and you get a medal. I would be competitions would call you and ask if you'd rather skate an exhibition, or compete against children if that was the only option. Or, since you aren't an adult yet, they might just group you in there.