Thomas was the third son and his elder brother was born before 30 Dec 1435. This says nothing about whether Thomas was born before or after that date and if we allow that it was after that date, there is nothing to indicate how long after although one can be certain that it was no later than 27 Apr 1467 although probably decades earlier.
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,
p. 323