“So this will be your 25th birthday,” a new acquaintance joked recently when he heard of my upcoming birthday.   There was that assumption again. When people — particularly women — get to a certain age, we’re supposed to lie about our age.     Certainly some people sign up for the game.   My own mother, born in 1936, still celebrates her 29th birthday. My friend Betsy, a devout Catholic, got to the age of 33 and decided to