Adding a bit to this, I think there's a good story here. The heir of Ballamillaghyn was an only son and likely only 14 when his father died in 1737. Not only that, he was the only legitimate grandson of his grandfather also named John Gelling who died only 4 years earlier. But grandfather John had an illegitimate son by Ann Kneale in 1705 whom he also named John. Both John Gelling Sr's wife Susanna Moore will of 1731 and Ann Kneale's will of 1714 mention this illegitimate child so he was clearly acknowledged. Illegitimate John married Ann Fayle about 1732 Marown (these marriages are missing, see Frances's Notebook as to why) and established a very strong clan of Gellings we've affentionately called the "Schoolteacher Gellings". John Gelling (of Ballig) was buried in Marown 1 Feb 1784 age 79.

Ann Moore, John G's widow of 1737, must have been very concerned because all she had to hold onto Ballamillaghyn was her young barely teen-age son John. So (my hypothesis) is that he married him off as soon as possible to Catharine Killey because male heirs were needed. All very patriarchal in those days. But it worked because a new John came in 1740 and by the time Ann Gelling als Moore died in 1848 she had 3 Gelling grandsons plus a granddaughter or two.

At his marriage in June 1739 John might have been only 16 or possibly barely 17 given his baptism of 23 Sept 1722 and assuming it was soon after his birth. He was 90 when he died in Sept 1812. I haven't run across any other bridegrooms that young in the Isle of Man, but maybe there were some others.