Most people over 40 have a particular admiration for the effortless style of the young. The baby boomers may have created the youth wave but their younger siblings grew up riding it and they never expect to topple off.Josh and Cornelia (Ben Stiller and Naomi Watts), the central characters of Noah Baumbach’s comedy While We’re Young, are fortysomethings who work in film. Josh doesn’t attract the recognition he would like as a documentary-maker. Then again, he has been making the same film for a dozen years, a lengthy study of the failures of the military-industrial system as expounded by a rambling academic. Cornelia, on the other hand, is producer of her father’s highly successful documentaries. Already, there’s potential for tragedy and comedy: Woody Allen t