In The World Until Yesterday, Jared Diamond notes that some traditional societies let small children play with and even suck on sharp knives. Diamond is not saying we should "emulate all child-rearing practices of hunter-gatherers." (That's good to know.) But maybe kids would learn some valuable lessons if we gave them a little more responsibility.

Which raises an interesting question: At what age should kids be allowed to use sharp knives? My six-year-old was trying to slice ravioli with a butter knife the other day and nearly gave me a heart attack. Do kids demonstrate that they're old enough to do something and then we let them do it, or are they simply old enough because they're doing it? Maybe age, like gender, is now just a social construct. The idea that there is a right age to use sharp knives or walk yourself to the bus stop or (looking to the future here) get married or have kids or start a career or move out of your parents' basement is … so 20th century.

That, anyway, is what I began to think after reading a couple of recent books from the crop of treatises purporting to explain Generation Y—people born between 1978 and 2000. In Twenty Something—Why Do Young Adults Seem Stuck?, Robin Marantz Henig and her daughter Samantha Henig, both journalists, offer a pop psychology tour of the scientific literature about so-called "emerging adults." They propose to compare millennials with their boomer counterparts on a variety of subjects to determine (at the end of each chapter) where "now is new" and where the behavior of the current crop of young adults is the "same as it ever was."

So, for instance, in a chapter about the way twentysomethings treat their brains and bodies, the authors conclude that "people still smoke too much and drink too much." Samantha Henig notes that when she got her first cavity at age 24, she realized that she had to start to worry about her body's "decay." Conclusion? "When young people are responsible for their own health, good habits go to hell." This is not exactly profound stuff. And while it may seem useful to compare boomers and millennials because boomer parents are often the ones wondering where they went wrong, we may also want to ask whether a comparison with the boomers is setting the bar a little low.

The Henig women mostly rely on studies by various psychologists, but they also put together their own survey, which they sent to friends and which was answered by 127 people. They don't bank on the results for any broad claims, but their anecdotes regularly draw from this survey. It becomes quickly clear that the Henigs' friends are a lot like them. In a chapter about marriage, they quote Michael, a 38-year-old engineer, who proposed to his girlfriend, "a graduate student at NYU who was doing her doctoral research on gender norms in courtship." In a chapter on career choices, the Henigs quote a 32-year-old woman who, prior to pursuing a career in architecture, tried out a variety of other jobs, including "small writing gigs, short-term consultancy, researching for professors, nannying." In other words, a reader would be forgiving for concluding that Generation Y are all college graduates from wealthy families who can't quite settle on the perfect mate.

To her credit, Hannah Seligson looks a little further afield for the millennials she profiles in Mission Adulthood. She includes a leader of college Republicans who grew up in the Mormon church, a veteran of the Iraq war who is also a single mother, and the gay son of Mexican immigrants whose attention-deficit problems are making it difficult for him to hold down a job and pay off his college loans. Though she acknowledges that everyone she picked has a college degree, there is still plenty of diversity here. Her "guiding question," she says, is "could we have met them a generation ago?"

But Mission Adulthood is still ultimately a defense of this generation against people who find them "lazy" or "stunted" or "entitled." Seligson concludes that these critics are the "victims of prejudice. They dislike and disdain what they see because they do not understand it." Maybe. But Seligson's case is not helped when we hear her subjects say "What people in the past might have gotten from church, I get from the Internet and Facebook. That is our religion." Or when Seligson describes "startup depression," the anxiety that comes AFTER you've succeeded in getting tens or hundreds of thousands of dollars in venture capital for your wacky new business idea.

Most people think that millennials looks different because they do everything later. They get married later and have kids later and settle on a career later and move out later. Why? Well, primarily because they can. Thanks to our longer life-span and modern technology, they have a lot more time to examine their choices. Both of these books go on at length about the paradox of choice and the related phenomenon of decision fatigue. Twenty-somethings are faced with too many options and exhausted by having to pick among them.

But they don't want to close anything off. The Henigs cite a fascinating study showing that this generation wants primarily to keep its options open. In a computer game devised by MIT psychologists, young adult players are given a certain number of "clicks" which they can use to "open doors" or—once inside a room—to get a small amount of money. After a few minutes of wandering, the players figure out which rooms have the most money. Theoretically they should simply keep clicking in those rooms. But sometimes, doors will start closing. Even players who know they will earn more from using their clicks inside a room will start to panic and click to keep the doors from closing. (The metaphor kind of hits you over the head, though, annoyingly, either the researchers didn't try this on older people to compare or the authors of the book failed to report it.)

So twentysomethings like to keep their options open. Oddly, in fact, they seem to be examining them earlier than previous generations (at least in recent memory). In the West, anyway, our helicopter parenting means that kids are thinking about what college they will go to when they're in elementary school. They are told from toddlerhood that they can be whatever they want when they grow up, and by the time they reach college they are paralyzed by the choice. The Henigs cite one young woman, a budding art historian who had not taken a science course in years suddenly agonizing about whether to take a college course called "Spanish for Doctors." "What if I want to become a doctor? Shouldn't I keep that option open?"

Millennials also starts engaging in sexual activity younger, which means that to the extent that they date, they will have 15 years of relationships with the opposite sex before they even think about marriage. Again, the options seem limitless. And finally, thanks to our early (and perhaps over-) diagnosis of psychological ills, kids start taking drugs like Adderall and Ritalin earlier and earlier. (The Henigs argue that prescription drugs are the new LSD. As of 2005, a quarter of a million college students were abusing prescription drugs.)

So what is the right age to get married and have children and buy a house and get a steady job and become financially independent? It may be hard to offer young adults a specific answer. But it is also possible to say that putting off decisions does not mean better results. The Henigs describe, for instance, the "slide" into marriage that happens when couples living together just decide it's easier to simply tie the knot rather than beginning the process of breaking up, dividing the stuff, etc. Later, they may slide into divorce.

Barry Cooper, a British author, recently warned in Christianity Today against worshipping "the god of open options." This god, Cooper said, "is a liar. He promises you that by keeping your options open, you can have everything and everyone. But in the end, you get nothing and no one." Good advice at any age.

Naomi Schaefer Riley is the author most recently of 'Til Faith Do Us Part: How Interfaith Marriage is Transforming America, just published by Oxford University Press.

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