How long has the term "situational awareness" been used in aviation?
According to M. Press, in his 1986 unpublished manuscript Situation Awareness, let's get serious about the clue-bird which is cited by many authors, the concept of SA can be traced back to aces of WW1, in particular Oswald Boelcke.

According to M. Press, his success was due to him being aware of the enemy before the enemy sees him.

From Situation Awareness Analysis and Measurement, Mica R. Endsley, Daniel J. Garland, p7:


  The earliest discussions of SA undoubtedly derive from the pilot
  community going back as far as World War I (Press, 1986)