Nonoy, thanks for the info on the Ifugao adze (and for the painting, too).
Back to the question on the probable age of the design of the Cordillera axe, perhaps one way to find the answer is to estimate the age of the shield's design.
For the two seem to function hand in hand:
"The Tinggian, Kalinga, and other northern tribes use the shield in combat at close quarters. The three upper prong projections are brought down violently against the enemy's legs so that he is tripped; when he is prostrate, the other or looser shield end with the two projecting prongs is brought down over his neck. The victim can then be effectively decapitated with the head ax that the head-hunter always carries with him." [Krieger 1926 96]
"With these tribes [Tinggian and Kalinga], it [the shield] is fashioned into three long prongs and two below... It has even an offensive purpose. In combat, the endeavor is often made to suddenly thrust the three prongs of the upper end against the opponent's legs and with a quick twist trip him up. As soon as he falls, the two prongs at the opposite end are jammed over his neck, pinning his head to the ground and allowing his easy decapitation." [Dean C. Worcester 1913, in National Geographic Society: 'The non-christian peoples of the Philippine Islands']."
Given that the victim can still be alive when the decapitation is made, the beheading has to be swift.
And a swift strike of the head axe will neatly do the job.
The samurai also used a shorter blade (wakizashi) when decapitating his fallen foe. The longer katana would have been impractical for the purpose.
Thus for me the institution of head-hunting means that just like the Cordillera shield, a short blade like the Cordillera axe must have been a very old design, given that the two appear to be two sides of the same coin.
PS - Per Krieger, the Igorot shield just like the Moro shield, has space on the hand-grab just for three fingers. The thumb and the pinky are made to rest outside the slot, to dexterously angle the shield when parrying a blow.