According to the International Genealogical Index (IGI) at
www.familysearch.org there were two couples named Peter Grant and Elspet Dow. One couple were married on 25 November 1824 in Knockando, Moray and the other on 21 February 1837 In Cabrach.
There were ten children of these two couples
First, both born in Knockando
Margaret, born 30 September 1825
Jane, born 19 May 1827
They almost certainly have nothing to do with yours.
Second, all born in Cabrach
Elspet, born 10 June 1837
Jean, born 6 December 1840
Alexander, born 3 February 1846
Isabella, born 19 April 1850
Margaret, born 19 January 1852
Janet, born 24 June 1855
Barbara, born 2 September 1858
Peter, born 23 April 1861
The 1841 census lists at Burntreble, Cabrach Peter Mitchell, 70; Peter Grant, 20; Elspet Grant, 20; Elspet Grant, 4; Jean Grant, 6 months.
At Alluie in 1841 are Peter Grant, 45, blacksmith; Elizabeth Grant, 20; and Jean Grant, 11. The original census records that they were all born in Cabrach, Banffshire, but the transcription at
http://www.freecen.org.uk/cgi/search.pl has changed that to Cabrach, Aberdeenshire.
In 1851, at Ardwell, Cabrach are Elspet Grant, 35; daughter Elspet, 13; and daughter Isobel, 11 months. Presumably Peter was working away from home, but I can't find him in the FreeCEN transcription. At Rhinturk, Cabrach are Alexander Dow, 83; his daughter Isobel, 23; his granddaughter Jean, 10; and grandson Alexander, 5, all born in Cabrach, Banffshire.
In 1861, at Rhinturk are Peter Grant, 44; wife Elspet, 45; daughters Elspet, 23; Margaret, 9; Janet, 5 and Barbara, 2. All born in Cabrach Banffshire.
In 1871 at Ballochford are Peter Grant, 54; wife Elspet, 55; daughters Elspet, 33; Isabelle, 20; Jannet, 15; Barbra, 12 and son Peter 9. Again, all born Cabrach, Banffshire
In 1881 Peter Grant, 64; wife Elspet, 65; daughter Elspet, 43 and Jessie, 25; and son Peter, 19; were at Burnside. Again, all born Cabrach, Banffshire. (Jessie is synonymous with Jan(n)et).
So it is reasonable to suppose that Peter was born in 1816 or early in 1817, and Elspet in 1815 or early 1816. It also seems reasonable to suppose that they were all born in the part of the parish that belongs to Banffshire.
As they both lived well past 1855, there will be death certificates for both at
www.scotlandspeople.gov.uk, where you can also view the originals of the above census transcriptions. You should also look at the birth certificate of Janet because 1855 certificates contain a lot of information not recorded in other years. The death certificates will tell you the full names of both sets of parents, including their mothers' maiden surnames, and you can use this to search for baptisms in the IGI and then check the originals at Scotland's People.
However don't believe everything you find in the IGI; the 'community indexed' listings are fairly safe but the 'community indexed' listings can be misleading. Use the IGi as a very useful pointer for finding the original records, but don't rely on it being correct all the time.
Now, to Peter senior. His age in the census is listed as 45. Ages in 1841 were supposed to be rounded down to the nearest 5 years, so he could in fact have been anything from 45 to 49, which, if it is accurate, would mean he was born between 1791 and 1796. There are a few Peter Grants born in Banffshire and Aberdeenshire between these dates, but none in Cabrach, so either he was born in a different parish or his baptism is not in the available records.
I can't see him in the 1851 census, which suggests that he probably died between the 1841 and 1851 censuses, so there won't be a death certificate, as these did not exist until 1855. I don't know the Genealogy of the Cabrach site, but I presume that the reason it just says 'Aberdeenshire' is because his actual birthplace is unknown.
I see that Peter Grant and Ann McPherson had two recorded children, both born in Cabrach, Aberdeenshire
James, born 25 July 1819
Elizabeth, born 11 May 1821
So perhaps this is the 20-year-old Elizabeth listed at Rhinturk in 1841. The death certificate of Peter Jr will clarify this.