There is a very good article in the Manx Museum Library called Manx Marriage Contracts: 1600-1736 by J. R. Roscow [IOMNHA Soc. Magazine Vol X No. 2 pp3-16]. You can buy a copy from them, and it is well worth reading. It explains "of age" and underage, etc.
"Of age" is different for wills and for marriage. If marrying under the age of 21 the person was supposed to have a parent's or guardian's permission.
Sue