If those ages are correct in 1851 it looks as though they were both underage at marriage, so it could be worthwhile looking at the marriage entry. Sorry, I don't have any Marown records, but someone may look for you in the MM Library.

In the 1861 census they are still in Toxteth Park, 25 Bk Parliament St. William Cottier 45, joiner, b.Isle of Man, Douglas, with wife Anne 49, Housekeeper, and daur Anne unm, 17, both also b.Isle of Man Douglas.

Sue