I agree with you that there would probably be a good reason for a man marrying at aged 17/18 when permission would be required. So the existence of a marriage contract is a good possibility, and will be easy to find if Braddan has the same format as the Northside parishes where the indexes are dated 6-monthly (in May & Oct.) from 1723 onwards.

For anyone interested in whether their ancestors had marriage contracts, and who don't have access to the MM Library, the LDS film numbers for the indexes are:
Index of Southside deed sales 1595-1847 : film no.106684
Index of Northside deed sales 1608-1847 : film no.106685

The marriage contracts can be easily spotted because the two names are bracketed together, followed by "Contract", then by the number of the contract.
As well as saying what each person will bring to the marriage (named land, money, goods, etc.) all four parents are often named - not just the fathers - and sometimes other family members are included as well.

Sue