"...the calculation begins with formation as an RFC or RNAS squadron..."
While it may be true that some RAF squadrons had their origins in either the Army or the RN, surely XYZ Sqn RAF cannot be any older than April Fools' Day, 1918?
Otherwise, for example, the Cdo gunners & loggies could say they were much older than they are, since the RM have had gunners and loggies since they were formed.
Oldest mil airfield - does Qinetiq count? Farnborough.
Does intermediate ownership count? - Upavon.