Janet married William Somerville before 12 Oct 1478 and as the minimum age for marriage was 12, her birth was before 12 Oct 1466.
Sources
↑Paul, James Balfour. "The Scots Peerage : founded on Wood's ed. of Sir Robert Douglas's Peerage of Scotland; containing an historical and genealogical account of the nobility of that kingdom", Edinburgh: David Douglas, 1910, Vol. VII, Archive.org,
pp. 116-7
↑ 2.02.12.2Cokayne, George Edward, "The Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom", London: William Pollard & Co, 1953, Ed. 2 Vol XII Part 1, FamilySearch,
9 p. 94-5
Robert Maxwell was married to Janet Douglas rather than this Janet Douglass who was still married to Alexander Gordon at the time that Douglas-3465 married.