I signed a TAA (Texas) lease agreement in December of 2011 which states at the top "This contract is only valid if filled out before January 1, 2010." Is it indeed invalid?

Depends.

Did you subsequently occupy the premises? Did you pay rent? Did the landord sign it?

Yes to all.

Then its valid

Yes, all parties were merely using this form to use as a contract. Since it was acted on and money paid, it is a legal contract.

If there are clauses that no longer meet state laws, those clauses may be held to be invalid