!I assume that there must be a record which states that John was 40 years old in 1531, and that this is where his birth date is derived from.
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file:///E:/E-S009/genealogy/Grab_A_Site_downloads/euweb/bzmisc09.htm
((A)) Sir John Bulmer of Wilton, Sheriff of Durham (d 25.05.1537) m1. Anne Bigod (a 10.1531, dau of Sir Ralph Bigod of Settrington) <constable01.htm> ((i)) Sir Ralph Bulmer of Sutton (d 09.10.1558) m. (before 01.10.1531) Anne Tempest (d 28.04.1555, dau of Sir Thomas Tempest of Holmside by Elizabeth Borough)
((a)) Joan Bulmer (dsp) m1. Francis
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m2. Margaret Stafford (d 25.05.1537, dau of Henry Stafford)