An 80-year-old man who was chatting with a friend on a Brooklyn street Thursday morning was mercilessly slashed across the face by a menace with a lengthy rap sheet, cops and sources said. The octogenarian and his pal were talking at Knickerbocker Avenue and Troutman Street in Bushwick around 6:30 a.m. when 56-year-old Israel Rivera allegedly approached the pair. "I'm going to cut you," he hissed before he knifed the older man across the face. The unidentified victim was taken to Wyckoff Heights Medical Center, where he was treated for the lacerations on his face.