I agree with Brian. Manx ages seem to be quite "fluid".
In 1841 most of the adult ages were rounded to the nearest 5 or 0.
Accuracy depended on who gave the info, who wrote it down, etc. Some of them actually have gotten younger as censuses have progressed

I have Robert as ch 15 Oct 1809 Lezayre s/o Robert Cottier and Elizabeth Stephen who md 22 Dec 1803 Ballaugh

1851 Ballaugh
, Ballaugh, 01-23, 101,
COTTIER, Robert, Intack, Head, Married, 67, Agric Lab, Lezayre
COTTIER, Eliza, Intack, Wife, Married, 65, , Ballaugh
CAIN, Eliza Bell, Intack, Granddau, , 8, Scholar, Ballaugh

Some time ago this line was being studied by
John & June Winchester