I am sure you will have picked up on the parish records via the Morman site, but for what it is worth: There are 2 Joan's born around the right time in Culmstock.One has a Christening date of 8 July 1744 (d of Christopher and Mary) , the other in 1755.The latter is the sister of my 4 x G Grandfather and I think married a Joel Wood.She would have been about 15 anyway at your marriage date.The former married James Stark in 1765, but of course the marriage to Absalom could have been her second. There was another Joan Christened 3.12.1752 but I believe she must have died in infancy.All these entries are extracted from the Parish records.
Finally, a long shot but it could explain the gap you are trying to bridge.A "Joan of Culmstock" was buried at the Prescott Chapel in January 1801.The chapel is just outside Culmstock and they hold quite extensive records of births and deaths. (knock on the door of the cottage next door!) There is also a copy of the Register at the PRO in London and I think they also have a copy at the Exeter RO.Perhaps that is where your missing birth is registered??Good luck!