Harley-Davidson Inc doesn't do much quietly. Its motorcycles are notoriously noisy. Its slogans - "Screw It. Let's Ride." - are loud too. So why was the Milwaukee company quiet last year when by its own numbers it successfully zoomed past a demographic hazard analysts had fretted about for years? Some background: In a recent interview, a top Harley-Davidson executive told Reuters that in 2012, for the first time in years, the average buyer of the company's bikes was not a baby boomer. For a brand defined by the emergence and, lately, the aging of the post-World War II cohort of consumers, that's a big deal - proof the 110-year-old company is gaining traction with a new generation of riders. Yet its top global marketing guru, Mark-Hans Richer, continues to insist it's no biggie - even though investors have long wondered how Harley would survive as boomers, who embraced its bikes as totems of rebellion in the 1960s and 1970s and drove its growth in the