My twin sons turned 18 and my Ex says I have "permanent" child support. Is there such a thing?

In 2006 here in Florida, I had a temporary modification in my child support from one amount to another higher amount until we went through the courts. The agreement for that higher amount was satisfactory and the court documents state that the amount was now be my "permanent" Child Support Payment. I have not missed any payments and the ORIGINAL divorce papers say I have my child support until my sons reach 18, which they have, of which one is living on his own. Now that my sons are no longer considered children, am I still responsible for paying my ex-wife child support because of the word "permanent." I'd rather control the use of their money for their college fund instead. Any guidance would be appreciated.