A void judgment is a term we use to describe a judgment that has been entered against you in violation of one or more laws. There are procedural laws and substantive laws. Procedural laws are those that require things be done a certain way or at a certain time. Substantive laws are laws that say what you can and can't do. Procedural laws are things like how long the creditor has to file a lawsuit, how long they have to serve it, who they have to serve, etc. Substantive laws are things like if you owe a debt you have to pay it back, if you are working they can garnishment your wages, and how much they can garnish.
In some cases, it doesn't matter how old the judgment is: if they broke the law, the court will set aside the judgment if you can show that certain laws were violated in obtaining it.