Marriage was not a civil affair until 1757 - prior to this Church held - that as Mona said rested on Roman Civil law (14 male and 12 female) - the church law was set out by Manx Chuch Synodal Statutes (latest was Bp Wilson's which required that both parties be confirmed which he also gave age of 14 for) - these were 'nodded' into Manx Law by the Governor etc.

Re child marriages - they were more common in England - have a look at http://www.ee.surrey.ac.uk/Contrib/manx/famhist/genealgy/marr.htm
and see if you can get a copy of Furnivall as he quotes many in Chester diocese - the children were 'bedded' together for one night and then seperated until they reached age of proper consumation.