I am the matron of honor in my cousin's wedding. We didn't know if I would be able to sign as the witness on the marriage license at the wedding, even though I am under 18. Does anyone know about this? Thanks :)

You would be a Maid of Honor at your cousin's wedding, as a Matron of Honor is a married woman. There should be no reason as to why you cannot sign as a witness, although individual state laws may apply. You can contact your local courthouse to be sure that you can witness the marriage.

Actually yes, many states ( that require witnesses) require them to be 18.
But then many states do not require a witness, the husband and wife signs, and then the preacher signs.
They should check with the clerk of the court where they get the marriage licese to be sure.

Wisconsin?
And yes, I'd be a maid of honor. Thanks for catching that :)