Hi
I have looked for Isabella Edington in Ancestry's index to the 1851 census of Scotland. They have a 57 year old Isabella living with her spouse Agnes, a 54 year old shoemaker and two children, Archibald, 26, and Ellen, 23. Unfortunately I do not have access to the film copy for Scottish censuses (an active subscription to scotlandspeople is needed for that) and so cannot check whether Agnes is or is not a mistranscription of James. The transcription gives England as the location for Isabella's birth. The census was taken on the 30/31 March 1851 so if the age is correct she was born between 30 Mar 1793 and 30 March 1794. I think that you would want to check the Hexham births for 1793-4 rather than 1791.
The 1841 census shows her 50 years old and married to James, a 48 year old shoemaker. The family includes Archibald, 18, Helen, 14, and three other young Edingtons, James, 11, John, 16 and Margaret, 20. The ages listed in the 1841 census are notoriously unreliable. The exact age was to be given for children but adults' ages were supposed to be rounded down to the nearest 5 or 10. The instructions about this obviously confused the enumerators and led to quite peculiar estimates and I think you are lucky if you can arrive at a birth date with a confidence interval no worse than +- 5 years.
You should note that Ancestry calculates the birth year as the census year minus the respondent's age but this is only accurate if the respondent's birthday falls between 1 Jan and the census date.
Good luck
Gobbo