As the marriage record to Mary Hill is silent on there ages, you will need to find his appropriate Christening record, the date on which is usually close to his birthdate, and will at least record his father's name. As you say, There are too many John Green's to be certain, although I think you should be focusing on his birth between 1760-1772. In my experience, the groom tended to be 20-30 years of age at time of marriage.