Wow, thanks to you all, it is a lot to digest.
My Mary Peterkin's birthdate guess comes from:
her marriage certificate:Sep 09, 1870 age 24
1871 Toronto census: age 27
1881 Toronto census: age 37
1891 Toronto census: age 45
her death certificate: 25 May 1896 age 51
She changes a bit but the range seems to be from 1844-1846, so a Mary found on the 1841 census would not be her.
I don't know exactly where she came from, but after having her first daughter in Toronto in 1873, she returned to Aberdeenshire to have her second son, James Ernest Peterkin Moir born Jul 29, 1878. She then went back to Canada before July 1880 when the next child was born.
There is a family from Old Machar, Aberdeen who showed up in Toronto for the 1861 census, after being on the 1841 & 1851 census of Old Machar. The parents are James & Mary & the children: William, Michael, Ann, James, Margaret, Mary(on the 1841 census) and Charles, Stuart, Robert and Marjory added on the 1851. The father and several of the children are in Toronto by 1861, so I wondered if my Mary went to join them as a relative. I don't see any Peterkins in Toronto who are not related to this one large family.
In 1861, they do have a 17 year old Mary living with them. She would fit my Mary, better than their daughter who would be 20 or 21 by that time. Relationship isn't given. However, they also have a 19 year old Annie, who should be 25, so maybe they are just lowering the age of their unmarried daughters. These 2 are positioned right in with their own boys, whose ages are also dropped a bit.
I will read things over and figure out what fits.
Thanks again for your great detective work.
Sharon