In the 1841 census "Isa" was 65 (assume age rounded) but husband John Brew's age was given as 72, so might be exact.

I wondered what happened to Isabella (Cain) Brew after John died, and this seems to be the answer:

1851 census transcript from FMP:

First name(s) Last name Relationship Marital status Sex Age Birth year Occupation Birth place
William Brew Head Married Male 41 1810 Woolen Draper Isle of Man, Isle of Man, Isle of Man
Sarah Brew Wife Married Female 36 1815 Woolen Draper Wife Cumberland, England
John R Brew Son - Male 11 1840 Scholar Liverpool, Lancashire, England
Edward R Brew Son - Male 9 1842 Scholar Liverpool, Lancashire, England
William R Brew Son - Male 7 1844 Scholar Liverpool, Lancashire, England
Thomas R Brew Son - Male 4 1847 Scholar Liverpool, Lancashire, England
Sarah J Brew Daughter - Female 2 1849 - Liverpool, Lancashire, England
Margaret Brew Daughter - Female 0 1851 - Liverpool, Lancashire, England
Isabella Brew Mother Widow Female 74 1777 - Isle of Man, Isle of Man, Isle of Man
Isabella W Brew Niece Unmarried Female 21 1830 House Servant Isle of Man, Isle of Man, Isle of Man
Hannah Tyson Visitor Unmarried Female 15 1836 Scholar Cumberland, England
Isabella Brew's Census Details
Details taken from the original census record.
First name(s) Isabella
Last name Brew
Relationship Mother
Marital status Widow
Sex Female
Age 74
Birth year 1777
Occupation -
Birth place Isle of Man
Birth town Isle of Man
Birth town as transcribed Isle of Man
Birth county Isle of Man
Street Williamson Street
Town Liverpool
Parish Liverpool
City Liverpool
County Lancashire
Hamlet -
Parliamentary borough Liverpool
Municipal ward Liverpool
Registration district Liverpool
Archive reference HO107
Piece number 2184
Folio 57
Page 29

An Isabella Brew died in Liverpool recorded June quarter 1856.

Sue