Correction:
I put the same baptism date for both Thomas's in the lists of children.

The first Thomas (s of Thomas C & Ann) should be 25 Jan 1748/49 Onchan. On his father's decree of Nov 1766 he is named supervisor of his two minor brothers William and Patrick, and he also was one of the inventory takers. Quite a responsibility for a 17-18 year old.

The Thomas bp 21 Dec 1745 will belong to Thomas C & Margaret because he is said to be "at lawful age yet too young to undertake the charge of the goods" in her decree of 1761. He'd be 15. This eliminates the 1748/9 Thomas for this couple because he'd be underage (about 12) in 1761. "Of age" is 14, I believe.

There is another Thomas bp 8 Oct 1752 to Thomas Christian in Onchan. He must belong to a completely different couple, because that would be too young to supervise anybody in 1766.

The value of the wills sure showed itself here.