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 t 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 ather 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 eugh to be a grouch. -You can speak from experience. -You w kw the things you wish you'd kwn back when, and still have time to do something about it. And while neither Ms. Zobel r 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 eugh wind) and beat their own drum (arthritis permitting).