You must both be over the age 18 to marry.  In certain circumstances a person who is under 18 (but over 16) may be given permission to marry.
Under no circumstances can two people under 18 years of age marry each other.
Authorisation to marry someone not of marriageable age
If someone under 18 wishes to marry he or she can apply to a Judge or Magistrate for an order authorising the marriage. The Judge or Magistrate will hold an inquiry to establish that the applicant has turned 16 and that there are exceptional and unusual circumstances that justify the making of the order.

Consent of the applicant's parents or legal guardians  will also be required.