on the 1841 census there is a Donald Mckenzie, shoemaker with a Bridget Mckenzie from Ireland, address is college wynd, Edinburgh, no idea who she is, he is listed as being 65 which would make him born about 1776, I'm guessing that the previous generation would be about 20ish when marrying, the first son Alexander was born in 1798 which would put Donald being born about 1778
Ah.
I think you might be at risk of making some assumptions here.
First, you would need to know whether there is any relationship between Donald and Bridget, that is, are they husband and wife, brother and sister (or cousins), or unrelated?
Second, adults' ages in the 1841 census, which was taken on 7 June 1841, were supposed to be rounded down to the nearest 5 years. So if Donald's and Bridget's ages are accurate, Donald would have been born any time between 8 June 1771 and 7 June 1776 and Bridget between 8 June 1781 and 7 June 1786.
Third, you cannot assume an age at marriage. A young tradesman would not usually have completed his apprenticeship and got hinself established to be able to support a wife and family much before his mid- to late 20s.
In 1851, at 14 College Wynd, is Bridget McKenzie, widow, aged 66, born Ireland. (The household includes a grandchild, Bridget Stamps, aged 14, of whom I can find no further trace.) Is she the Bridget McKenzie who was buried in Canongate in 1853 aged 71, and is she also the Bridget Brannan McKenzie, same age, same year, same parish?
There are several burials of Donald M(a)cKenzies in Edinburgh between 1841 and 1851 - I see that all the Canongate ones are recorded twice. Do any of these records give any additional information, for example an occupation or a burial matching that of Bridget?
Similarly there are numerous Mary M(a)cKenzies who were buried in Edinburgh between 1804 and 1841, including at least one (in 1814), with no age recorded, linked to Dond McKenzie.
Do any of the children's marriage records mention Donald, and in particular whether or not ne was deceased?