Hi aga.p
All the information I have shows that Donald was born in May of 1839. The 1851 census has him as 12 years, 1861 as 21 and his siblings Mary, John and Margaret are younger than him, then in 1871 he appears as 28, he's a boarder, his parents have passed and his age stays younger from then. So I tend to go with the first census rather than the later.
The link you attached for Donald has me confused on his death date, I have Sept. 25, 1919 as it was carved into his headstone. Could you tell me if you have his death certificate to verify the date you have.
Donald is my 3x great uncle, his older brother John is my 2x great grandfather. There is only one sibling that we can't trace after their mother's death, perhaps you have information on Catherine (Christy), she was born 1851. I didn't find any death certificate for her but it's been years since I've actually worked on the Buchanan tree.
I also heard that a few of Donald's family members made trips back to Benbecula so any information you have would be wonderful.