KIAWAH ISLAND, S.C.—Among the congratulations that poured in for Phil Mickelson becoming the oldest major champion in golf was a video tweet from Jack Nicklaus, who is still good at math. “You know, something sort of strikes me that 50 years old is older than 46,” Nicklaus said with a grin. “Well done, my friend. Many more.” Many more? At age 50? Mickelson plays a game with which Nicklaus might not be familiar, even though the most iconic of his 18 majors was Nicklaus winning the Masters at age 46. During an interview at the end of 2016, Nicklaus said that sixth green jacket “was an accident in many ways.” He started to scale back his schedule after winning two majors in 1980. He won twice more before the 1986 Masters, but he really wasn’t playing much golf. “It’s really difficult when you’ve had as much success as I had over a …