Other than a few special guest star appearances, she didn't return as a main cast member until season 12.
That's why a lot of women leave those procedurals.
While she was a first year student, she debuted as an actress and eventually dropped out of design school.
As is sometimes the case with network procedurals, the real reason is more complicated than it seems at first glance.
Hotchner is immediately suspicious of the new agent, but Prentiss, insisting that her mother had not pulled any strings for her to get the assignment, convinces him to let her join the team on a probationary basis, and eventually proves her competence and gains the team's trust.
At least someone can be attracted to an older woman in Hollywood! She graduated from Chesapeake Bay University with a B.