The statue of limitations is the amount of time the State has to file charges against someone and starts from the date of the alleged criminal violation. For misdemeanors, this is two years (TX Code of Criminal Procedure 12.02).
Your situation seems to be that you were issued a ticket 20 years ago and never took care of the case. There is not a statute of limitations issue here since the State seems to have filed the charges about 20 years ago. There may be a speedy trial issue if the warrant was inactive for 20 years, but if you have just avoided detection for that period, then you may not be able to use a speedy trial defense.
You need to speak to local criminal defense attorney about how to take care of this matter.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.