The Maughold marriage of John Corkil & Ellinor Casement on 3 Feb 1781 was with consent of parents, which indicates that both were under the age of 21.

Elinor daur of Robert Casement & Anne Curphey bapt Maughold Parish Church 8 Aug 1762, as you have.

John Corkil could be John Illegit son of William Stole & Margaret Corkil bapt Maughold Parish Church 23 Mar 1760, or this John could have been known as John Stole, even though illegitimate.

If it was only Elinor who was under age another possiblility for John was John bapt 11 March 1759 Maughold son of Peter Corkil & Isabel Kneal.

By the way Robert son of John Corkil & Ellinor Casement bapt Maughold Parish Church 20 Dec 1784 was a twin "Private baptism. Twin of John".
John son of John Corkil & Ellinor Casement bapt Parish Church 20 Dec 1784 Private baptism. Twin of Robert.

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