The baby boomers are turning 50 - one by one, at the alarming rate of every seven seconds for the next 16 years. And just how many will be happily looking forward to blowing out a galaxy of candles and admitting to the world that they're half a century old? Very few, in Allia Zobel's opinion. But that's because they're not looking on the bright side. In a new collaboration after The Joy of Being Single (138,000 copies in print), Ms. Zobel and New Yorker cartoonist Roz Chast take on another seemingly sore subject - being 50 in society that worships youth - and find excellent reasons to celebrate. Such as: -You can get a tattoo and chalk it up to hormone imbalance. -You can brag to your friends if a construction worker whistles at you. -You can have complaining contests about your bunions. -You look divine in hats and sunglasses. -You're old enough to be a grouch. -You can speak from experience. -You now know the things you wish you'd known back when, and still have time to do something about it. And while neither Ms. Zobel nor Ms. Chast is anywhere near the magic five-oh (it's the fluorescent lighting), they speak with authority as they advise all 50-year-olds to blow their own horn (if they have enough wind) and beat their own drum (arthritis permitting).