You might assume that this is a strictly biological phenomenon. But whether the timing of the change from adolescence to adulthood is genetically preprogrammed from birth or set by experience (or some combination of the two) is not known. Many studies find a marked decline in novelty-seeking as we move through our 20s, which may be a cause of this neurochemical shift, not just a consequence. If this is true — that a decline in novelty-seeking helps cause the brain to harden — it raises intriguing questions about whether the window of adolescent brain plasticity can be kept open a little longer by deliberate exposure to stimulating experiences that signal the brain that it isn’t quite ready for the fixity of adulthood.

Evolution no doubt placed a biological upper limit on how long the brain can retain the malleability of adolescence. But people who can prolong adolescent brain plasticity for even a short time enjoy intellectual advantages over their more fixed counterparts. Studies have found that those with higher I.Q.s, for example, enjoy a longer stretch of time during which new synapses continue to proliferate and their intellectual development remains especially sensitive to experience. It’s important to be exposed to novelty and challenge when the brain is plastic not only because this is how we acquire and strengthen skills, but also because this is how the brain enhances its ability to profit from future enriching experiences.

With this in mind, the lengthy passage into adulthood that characterizes the early 20s for so many people today starts to look less regrettable. Indeed, those who can prolong adolescence actually have an advantage, as long as their environment gives them continued stimulation and increasing challenges.

What do I mean by stimulation and challenges? The most obvious example is higher education, which has been shown to stimulate brain development in ways that simply getting older does not. College attendance pays neural as well as economic dividends.

Naturally, it is possible for people to go to college without exposing themselves to challenge, or, conversely, to surround themselves with novel and intellectually demanding experiences in the workplace. But generally, this is more difficult to accomplish on the job than in school, especially in entry-level positions, which typically have a learning curve that hits a plateau early on.