“Put off today what you can do tomorrow” has long been the motto of many baby boomers. Until, that is, the biological clock began its inexorable countdown. Today even some of the most committed postponers of parenthood are finally deciding to have children, producing a record crop of late-in-life babies. The number of women 35 or over who are giving birth for the first time has quadrupled in the past decade, and is expected to increase further in the next few years. Sure, there are advantages to starting a family in your late 30s and early 40s. But what about the children who must build sand castles with graying oldsters who can’t play ring-around-the-ro sy without breaking into a sweat? In some U.S. urban areas, older parents are becoming the norm. Author Martha Fay, 41, mother of a five-year-old daughter, says of her West Side Manhattan neighborhood, “Some of the mothers look so old they don’t appear biologically capable of having had these children. We have 50-year-old men teaching soccer teams.” For both sexes, the benefits of postponing kids are greater financial security and well-established careers. What is more, there is no question that late children are wanted — often badly wanted. Says Susan Fillin-Yeh, 45, an art historian at Yale and mother of a nine-year-old daughter: “At this stage I’m not battling to find out who I am. I’m a better parent now than I would have been.” That may be. But there is plenty of evidence that late children often have problems that other kids do not face. Witness Last Chance Children , a new study of 22 adult children of older parents by sociologist Monica Morris of California State University, Los Angeles. Morris found that only two of her subjects would wholeheartedly choose to have their children later in life. The others unleashed a litany of lateborn woes. They said older parents, usually fearful of physical injury and health problems themselves, were often reluctant to participate in games and sports. Some complained they were deprived of grandparents at too early an age. “No doubt, having children earlier is better and later is worse,” says Yale Psychologist Edward Zigler. “Children are always a blessing and a trial.” It is no secret that children may be embarrassed by their parents’ gray hair, their outmoded clothes and opinions that may seem as antediluvian as dinosaurs. And parental physical incompetence can be mortifying. For Tom McDonough, 49, the memory of playing baseball with his 58-year-old father is especially painful. “I said, ‘Dad, run, run.’ He dropped the bat and looked at me and said, ‘I can’t.’ ” Says Sasha Lawer, 30, a daughter of older parents: “When I wanted to play, they would send my brother.”