Are you referring to the William Teare that IGI has at Oak Grove, Pierce, Wisconsin in 1880? The suggested YOB there is 1819. (Could also be 1820).

There are 8 William Tear/Teare's in the Christening/Birth records on IGI for 1818-1820.

Assuming you are certain that he was born 1818, then that cuts it down to just 2: The one shown as born 20 Jan 1818, who married Esther Quirk, put there is no parish information; or, the one christened at Kirk Andreas on 25 Dec 1818, son of Wm. Teare and Ellin'r. Crow.

If you can find out where your William was baptised/christened, then you could pay a researcher to look through the baptismal records for the relevant parish for perhaps a decade either side of 1818 in order to track down all the siblings. You could then look for these on the 1841, and subsequent censuses.

If you have John's YOB then perhaps you could pick him straight out of the census. There is an online IOM census index at:

www.iomfhs.im/lawsons/LawsonBMD/census/census_index.html?Submit=Census+Indices