Robert was married the second time in 1457 and one can allow that his first marriage was likely long before 1456 but using 1456 as a basis, one can infer his birth was before 1442. However, his daughter, Christian, married on Feb 1451 placing her birth probably prior to 1430) and the birth of her son Robert places her birth before 1424 and this Robert's birth prior to 1409.
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, 1906, Vol. III, Archive.org,
p. 221
↑ 2.02.12.2Paul, 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, 1906, Vol. III, Archive.org,
p. 223
↑Cokayne, George Edward, "The Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom", London: London, St. Catherine Press, 1926, Ed. 2 Vol V,
p. 105
↑ 4.04.1Paul, 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, 1906, Vol. III, Archive.org,
p. 224