Pretty Little Liars executive producer I. Marlene King really loves her red herrings. While fans continue to try and figure who A.D. is, King keeps bringing in characters with the those initials. First it was Archer Dunhill then it was Addison Derringer followed by Sydney calling herself an Anonymous Donor. It's clear King is trolling fans but fans can't help but wonder is Addison really just another red herring or is she someone fans should be giving a closer look. Unfortunately, there's no one theory that proves Addison isn't A.D. on Pretty Little Liars but there's a few good reasons why she probably isn't.
For one, she's too young. Yes, Mona was also in high school when she was A and Ali was a force to be a reckoned with as a teen, but King makes A.D. out to be someone that's had a grudge against the Liars for years. Therefore, even though Addison is in high school now she would've had to be in middle school or even elementary school to have planned out the Liars torture for this long. That just doesn't make sense.
Moreover, although there's definitely plenty of high school geniuses who could've made the board game, I seriously doubt Addison's one of them. The girl ditched swim practice just to smoke with her boyfriend. She doesn't seem like the type of person to dedicate her free time to making this insane board game for these five women, who as far as we know, she doesn't even have any connection to.
Also, speaking of connections, how could Addison have kidnapped Jenna and made her do her bidding? Addison is tiny and again a teen. You mean to tell me this teenager was pulling the strings on not just Jenna but probably Noel and Sara as well? That seems highly unlikely. Plus, again, why would these adults do what Addison asked of them anyway? What could she possibly have on them? She couldn't even successfully get Emily fired so how could she do the level of scheming that A.D. does. Easy answer: she can't.
Lastly, many fans believe Addison actually has nothing to do with the show's plot but instead is the starting point for a Pretty Little Liars spin off. That's how fans explain why she's listed as appearing in the series finale on IMDb. She'll come in at the end to mark the start of the next generation of Liars. It's why she's so much like Alison; she's supposed to be the new Ali.
While it definitely seems impossible that Addison is A.D. fans will just have to wait until the series finale to know for sure.