Malcolm Young, the AC/DC co-founder whose brash guitar riffs, tight songwriting and live-wire personality defined an era of rock Ďní roll, has died. He was 64.AC/DC announced his death on its Facebook page Saturday. The time, location and cause of his death were not made public. Young had been diagnosed with dementia in 2014 and did not perform with the group on its recent comeback tour."Renowned for his musical prowess, Malcolm was a songwriter, guitarist, performer, producer and visionary who inspired many," the band said in its statement. "From the outset, he knew what he wanted to achieve and, along with his younger brother, took to the world stage giving their all at every show. Nothing less would do for their fans."Over 40 years, AC/DC would sell more than 200 million albums of riff-driven, sometimes juvenile but always anthemic rock, including 1980ís "Back in Black," which remains the second-best-selling rock album of all time.Born in Glasgow, Scotland, in 1953, Young moved with his family to Sydney, Australia, in 1963, and formed AC/DC with his brother Angus in 1973 after watching older brother George find success with his group the Easybeats. Though Angus, with his schoolboy costumes and flamboyant lead guitar work, was the de facto frontman, Young was its musical core and primary writer for most of the groupís hits, which included stadium-rock staples such as "Highway to Hell" and "Back In Black."His exacting, blues-driven guitar work was the foundation for AC/DCís sound, and Young was widely considered one of rockís greatest rhythm guitarists. "Malcolm Young was one of the best ever rhythm guitarists in rock Ďní roll," said Guns Ní Roses guitarist Slash in Rolling Stone. "He was a fantastic songwriter and he had a great work ethic too.""It is a sad day in rock and roll," Eddie Van Halen said in a Twitter post. "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."After settling in with drummer Phil Rudd and bassist Mark Evans (later replaced by Cliff Williams), the group scored significant hits early on with singer Bon Scott. The act debuted in 1975 with "High Voltage," followed by a string of popular records including "T.N.T.," featuring fan favorite "Itís a Long Way to the Top (If You Wanna Rock Ďní Roll)," later that year. Early success continued with 1976ís "Dirty Deeds Done Dirt Cheap" and 1977ís" Let There Be Rock."The actís 1979 LP "Highway to Hell" cracked the Billboard top 25 and went gold in America. But its future was thrown into doubt after Scott died in 1980 after an all-night drinking session.The band soon recovered, bringing on new singer Brian Johnson. AC/DC quickly wrote what would be the defining LP of its career, 1980ís "Back in Black." The LP, whose cover was an all-black tribute to Scott, sported some its most popular singles, including "You Shook Me All Night Long," "Rock and Roll Ainít Noise Pollution," "Hells Bells" and the title track. Its 1981 follow-up, "For Those About to Rock We Salute You," continued its hot streak, topping album charts in the U.S.The hard-partying group saw some controversy during its ascent. While songs like "Beating Around the Bush" and "Big Balls" were tongue-in-cheek barstool jokes, conservative activists accused the band of satanic imagery (including the rumor that its name stood for "Anti-Christ/Devil Children" - the group members actually got it from the voltage on back of their sisterís sewing machine). The Los Angeles serial killer Richard Ramirez was a notorious fan, leaving an AC/DC hat at the site of one of his murders."Part of AC/DCís appeal to its predominantly young, male audience is the bandís flashy, high-energy show and the hell-raising bravado of its lyrics," the Los Angeles Timesí Robert Hilburn wrote in a 1982 profile of the band. "But it also helps that the group makes adults uneasy. Much the same way rock extremists as Jimi Hendrix, the Rolling Stones, the Doors, David Bowie and Alice Cooper did earlier."AC/DC remained one of the worldís biggest rock bands. The bandís hard-drinking lifestyle, which had already claimed Scottís life, again caught up with the act when Malcolm announced that he needed to temporarily leave the group to get sober. He was briefly replaced by his nephew Stevie Young, who would later again replace Malcolm after his dementia diagnosis.Malcolmís return on 1990ís "The Razorís Edge" helped make it a multiplatinum smash in the U.S., selling 5 million copies, and its subsequent tour was documented on the concert album "Live." In 2003, the group was inducted into the Rock and Roll Hall of Fame, and again topped U.S. charts with its 2008 LP "Black Ice." AC/DC won its only Grammy in 2009 for best hard rock performance for "War Machine." Malcolm last performed live with the group in 2008.In 2014, after his dementia diagnosis, Young announced he would permanently leave the group. They recorded one final album, 2014ís "Rock or Bust," without him, and his absence weighed heavily on the group as it prepared for a comeback tour, including a headline date at the Coachella Valley Music and Arts Festival in 2015."You pick yourself up, dust yourself down and just keep going," singer Brian Johnson told The Times in 2015. "You live on, and you have a wonderful memory of them always with you, but youíre not going to stop doing what you do. Otherwise, you die inside, you know? And we would die - I would, if I didnít do what I was doing. Thereíd be nothing."Young is survived by his wife OíLinda, children Cara and Ross, son-in-law Josh, three grandchildren and his sister and brother.