I looked up a few US state requirements, at random. It really is all over the place.
Florida, Arkansas, Virginia, Illinois, Maryland, New Hampshire. No witnesses required.
New York City. At least one witness, over age 18.
Washington. Two witnesses, no age requirement, but over 18 strongly recommended as they are witnessing a civil contract.
New Jersey. One witness, over age 18. The witness has to be there when you apply for the license.
Nebraska. Two witnesses, no age requirement.
Iowa. Two witnesses, over 18, personally known to the couple.
Rhode Island. Two witnesses, over age 18. The state notes that if "honor attendants" are under 18, someone else has to sign.