Goldie, yes, Millar is a stumbling block for almost everyone researching this family. Some of the confusion about his age comes from a mistake in the 1851 census, where Millar and Margaret are both listed as 10 years older than they actually were (the kids ages were correct). Here are the census listings I have for them:
1841 (June 7) Millar is listed as 33, Margaret as 30
1851 (Mar 31) Millar is listed as 56, Margaret as 55 [clearly in error, they were 10 years younger]
1861 (Apr 7) Millar is listed as 57, Margaret as 56
1871 (Apr 2) Millar is listed as 68, Margaret as 66
1881 (Apr 3) Millar has passed, Margaret as 77
Ignoring the 1841 ages which are inconsistent, and correcting the 1851 ages, Millar would have been born about 1804, give or take. In the 1861 census Margaret is listed as having been born in Govan, and I have found a Margaret Robertson born in Govan, Lanarkshire, on 1804 Apr 13, which is just about right. Millar is listed on the census as having been born in Renfewshire. Although no specific place is mentioned, he married, had kids, and died in Abbey, Paisley, so it is likely he was born there as well. The Abbey seems to have kept good records. Maybe someone here has access to them. Hope that helps!
Jim