The Library and Archives Canada has updated the 1851 (1852) census.
The 1851 Census marked the second collection of statistics for the Province of Canada (consisting of Canada West and Canada East). Information was also collected for New Brunswick and Nova Scotia.
In addition to searching by geographical information such as province, district, and sub-district, users can now also search by nominal information such as name, given name(s) and age of an individual.
In Canada East and Canada West, the census was supposed to have been taken in 1851, but was actually take in January 1852.
So, in the Canada East and Canada West, it will be the age of the person's next birthday in 1852, not in 1851 (Column 6).
Also, in Canada East and Canada West, there was an urban and a rural census, and they asked different questions.
In Nova Scotia, and New Brunswick, the census was taken between June and December 1851.
If you are having difficulty finding the person you are looking for in the 1851-1852 census, not all schedules survived.