Unlikely. Review YouTube terms of service. Contact an Intellectual Property attorney.
My comments have been made without discussion. An attorney client relationship has not been established. There may be conflicts which prohibit my providing you with specific legal guidance. Any contact with you beyond these few general words will start with a disclosure of opposing parties so that a conflict check can be made. You should discuss with an attorney.
"Ex post facto" is making illegal (or increasing the penalty for) conduct that has already occurred. What YouTube may have done in 2009 under 2009 terms of service probably don't apply to actions under a new set of rules in 2014.
Second, there is the issue of standing. How exactly has the "2014 users" class been hurt by the non-blocking of 2009 videos that allegedly violated someone else's copyright? I don't see it, especially if YouTube changed its terms of service in the meantime.
Under federal law, service providers like Google and YouTube usually have a safe harbor, whereby they are not liable for not taking something down unless someone complains about it. If no one complains about a video, or fails to back up a complaint with evidence, then YouTube has the right to continue posting the video.
Finally, someone who is infringing another's copyright is probably not in a good position to complain that others' infringing content was not taken down. Unless some bias can be proved (e.g., YouTube takes down all videos about one sport, but not for another sport), the argument won't go very far in a court of law.
This answer does not create an attorney-client relationship. It is not legal advice. Please contact a lawyer qualified in your jurisdiction to discuss your situation in confidence, using your factual details. Avvo answers are only general legal responses. Item 9 of Avvo.com's Terms and Conditions are incorporated in this disclaimer as though it were printed here.
That is hardly a stumper. The statute of limitations is three years, so no suits based on Moe than five year old causes of action are likely DOA. Also, DMCA answers that, and immunized YouTube. That suit would go likely nowhere.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.