The minimum age for a witness at a wedding varies depending on the state and country, but in Australia it is 18 years of age. However, the New York State Department of Health website does not specify a minimum age requirement for the state.
The witness to a wedding needs to be carefully chosen because he may have to testify in a court of law as to what he saw. Usually a witness is a well-known friend of the couple getting married. The number of witnesses required from a legal standpoint is different in each jurisdiction. In Australia, two people are required as witnesses, while in New York State there needs to be at least one witness. Witnesses typically must not only observe the ceremony, but are also required to sign the Marriage Register and Certificates.