There is no point in taking a freshman.
A sophomore could go just to see what’s our there.
A junior should be attending, asking lots of questions and talking to reps from schools of interest.
A Senior should be attending and checking in with reps of schools they plan to apply to.
My daughter is pretty shy. These big busy events were a great way for her to get used to talking to college reps without feeling like all eyes are on her. The schools she applied to had interviews, and she ran into some familiar faces after talking with reps at college fairs. Keep in mind, some colleges will send the actual area admissions rep that will be reading your child’s application, while others will send a local alum. For smaller colleges, putting an interested and kind face to the name they will see on an application can sometimes tip things in your favor.