I found a death certificate for a Donald Frederick Stuart (same name as youngest son) from 1948, that says Donald was born in 1901. His parents are given as Herbert Clark Stuart from New Brunswick and Faye Coldwell from Nova Scotia. Assuming that the DOBs for the parents are correct, this COULD be the correct son, and the age would also fit quite ok. Also Donald is said to be born in St. Louis, Missouri.
"Nova Scotia Deaths, 1890-1955," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:QK99-B14J : 8 November 2017), Herbert Clark Stuart in entry for Donald Frederick Stuart, 09 Dec 1948; citing Halifax, Halifax, Nova Scotia, Canada, certificate 6214, Nova Scotia Archives, Halifax.