You would have a tough case against the seller.
The Seller's Disclosure states that it provides information based on a the seller's "knowledge and belief" and expressly states that it is not a warranty of any system. Both the Sellers Disclosure and the typical Realtor® form contract provide that the Buyer is permitted to make inspections and demand repairs - but otherwise the home is sold "As Is". Thus, unless you can prove an actual fraud - i.e., an intentional lie as opposed to an incorrect guess as to the age of the unit - it is unlikely you can recover from the seller.
Did you have a pre-closing home inspection by a licensed home inspector?
What did your home inspection report state about the age and condition of the AC system?
This answer is for general education purposes only. It neither creates an attorney-client relationship nor provides legal guidance or advice. The answer is based on the limited information provided and the answer might be different had additional information been provided. You should consult an attorney.