For those of you that think AJ took off voluntarily, please consider what that involves, and the amount of adeptness required to stay hidden from LE and the FBI:
AJ would have to:
Not care about leaving her friends and family
Be OK with causing fear and suffering to her friends and family
Be absolved to wasting tuition, missing exams and derailing her academics
Be selfish enough to disappear herself and not help her sisters (if the reason was home/safety)
Have money, connections and means of living and not being traced which takes planning - in conflict with leaving impulsively after a fight etc.
Not care about wasting LE resources to find her
Have a grave need to disappear, such as having committed a crime, or fear for her life, in which case she must also have a reason for not using LE to help solve her problem.
Have at least one accomplice - at 18 I have little doubt she could successfully disappear without someone's help, and that someone would have to be so disconnected from her life that LE hasn't been able to trace her by now.
At 18 and a college student, AJ is covered by Suzanne's Law, so there is no doubt LE took this seriously from the beginning, investigated it as a missing persons from the beginning, and were always concerned/on high alert as soon as family dynamics were figured out with the help of WH. Their behavior and investigation patterns now tell us this is unlikely a voluntary missing case. While we can't take his timeline as fact, there is plenty more about WH that also points away from AJ being gone voluntarily. He says himself he thinks she was abducted by "someone" and we know he likes to refer to himself in the 3rd person as "someone" quite often.
She can't be a runaway, because she is an adult. She has her own dorm/apartment and her own transportation, two other parents and a huge network of friends. If she was free to leave voluntarily, she was free to go anywhere to get away from her family to feel safe that did not need to involve staying dark long enough to launch an investigation. Keep in mind that Amazing Amy in Gone Girl is a sociopath. AJ doesn't fit the profile of someone that would fake her own disappearance for revenge or attention.
I am sure I am not alone in hoping AJ proves me wrong and shows up.