there are two conflated ideas - a legal marriage could be contacted at age 14 for males (12 for females) though many of these child marriages did result in divorces (for various reasons) - some of this is also on my website there are some Manx Marriage contracts that the children will marry when at age.
However as Sue points out parents and/or guardians had to agree if marrying under 21 unless previously widowed - out of birth parish marriages might have bent rules a little in that one party might be just under 21 but the usual requirement was to provide a certificate from the parish of birth stating they were of age to marry without consent