The Los Angeles Chamber Orchestra hits the Big 4-0, but it’s not worried about wrinkles or spare tires as it gets ready for an anniversary gala harking back to the good old days. First off, there’s Sir Neville Marriner, the legendary founder of St. Martin-in-the-Fields, who was LACO’s first music director (1969-1978), and who returns, at age 84, to conduct a program that includes a work by the orchestra’s first composer in residence, Paul Chihara — an arrangement of Schumann’s charming Scenes From Childhood. Then there’s LACO’s current music director, Jeffrey Kahane, who’s tickled to death to be performing as soloist in Beethoven’s Piano Concerto No. 1 under the baton of his longtime idol. “When Sir Neville gave the downbeat at LACO’s first concert on October 27, 1969, I was a 13-year-old aspiring piano student,” Kahane recalls. “It is with particular joy that I welcome Sir Neville back to the podium and take my place at the piano, fulfilling a lifelong dream.” The concert and celebratory champagne toast promise to be fun enough, but for those with a few more bucks — say, 750 of them — there’s a Ruby Jubilee Celebration afterward that includes a gala for Sir Neville, dinner, dancing and a live auction. Happy anniversary, LACO, and here’s to at least 40 more. Ambassador Auditorium, 131 St. John St., Pasadena; Sat., Sept. 27; celebratory toast 6 p.m., concert 6:30 p.m.; $30-$125 concert only; $750 Ruby Jubilee. (213) 622-7001, Ext. 215, or www.laco.org.