In a relatively recent developement in Florida law, fathers (in paternity/child support cases) can challenge the determination of paternity until the child's 18th birthday. Of course, there are some conditions and limitations on the ability to make such a challenge.
The conditions / limitations:
1. Father must have DNA test or has requested one,
2. Father must be current in child support (or have "good cause" as to why he is behind)
3. Father cannot have agreed he was the father after knowing that he was not the father.
There are a few other technical conditions that I won't get into here.
The statute that allows for this paternity challenge is contained, as you might expect, in the paternity statute (Fla. Stat. 742). Whether or not these provisions apply to ex-husbands is less clear. There are some indications that ex-husbands who were lied to regarding the paternity of their children may have the same protection that fathers in paternity cases have. This issue has not yet been decided by the appellate courts in Florida (but I'll bet it will be soon!).
The bottom line is this: If you are paying child support for a child that you think or know is not your child, the law now allows you to prove you are not the father and, as a result, stop the child support.