Trixie was too busy filing one of her hoofs to really pay attention, given that she knows she'll win. However, when she takes a little glance, all that happens is Trixie's mouth falling wide open.

How could Twilight possibly do an age spell!? ...Wait a minute, why is Trixie surprised. She still has a lot more powerful spells that she can use to prove her superiority.

"Eh... Ho hum, so you can do an age spell. Big deal."