A 1798 Letter of Consent for a Son to Marry

In 1798, John Sledd gave permission for his son, Thomas Sledd to marry Sally Tinsley, daughter of John Tinsley, and directed his letter to the Amherst County official who oversaw marriages. The letter is strong evidence as to the names of the fathers of Thomas Sledd and Sally Tinsley. Based upon the writing it even appears that John Sledd wrote out the consent himself.

What I need to determine if that if the consent was required for males who were not of legal age to marry in 1798 as that would help me pinpoint down when Thomas was born.