This morning, the world learned of the death of AC/DC founder guitarist Malcolm Young at the age of 64. He had been suffering from dementia for many years. Many of his friends and admirers shared their grief on social media. On Twitter, Eddie Van Halen wrote, "It is a sad day in rock and roll. Malcolm Young was my friend and the heart and soul of AC/DC. I had some of the best times of my life with him on our 1984 European tour.