Milestones can be perceptually important.
It doesn't matter if that milestone is "real" in the sense of being physically or practically significant: our perception of a milestone is what's important.
Thus, while the winter solstice might be a real-deal physical event that some of us notice and commemorate in different ways, our birthdays and the celebration of culture- or industry-invented holidays can be just as or more influential on our mood, our sense of meaning, and our experience of time.
We're capable of imbuing all sorts of things with significance, and that imbuing is what seems to be most impactful when it comes to milestones nudging us toward certain behaviors.
Recent research suggests, for instance, that when people are approaching a new decade—will soon turn 20 or 30 or 40 and so on—they're more likely to take stock of how meaningful their life is and has been, and are more likely, as a consequence, to make substantial changes to how they're living, where they're living, and how they're spending their time.
This meaning-assessment tends to occur in the year leading up to that age-milestone, and this tendency was originally noticed by people researching the abundance of greeting cards aimed at "nine-enders": people who are ages 19, 29, 39, etc, and who are thus on the precipice of a decade-milestone birthday.
This research found that 29-year-olds are about twice as likely to run a marathon (compared to 28-year-olds and 30-year-olds), and that 49-year-olds are three-times more likely to run a marathon than 50-year-olds.
People are also more likely to post a faster finish time than those two-years older or younger if they're 29 or 39 and had previously run a marathon.
It also found that cheating on one's partner (most common at ages 39, 49, and 29, respectively) and suicide are more common amongst nine-ender cohorts, but folks in these age-groups are also more likely to focus on tangible health-outcomes (like their body-mass index) and general sense of health satisfaction, over daily positive emotions (like those associated with relaxing rather than working out; more big-picture thinking, fewer casual, emotion-driven behaviors).
The general idea here is that people seem to search for meaning in their lives as they approach an age that seems—to them—like a milestone.
And that makes a sort of intuitive sense, even if the research hasn't always been as solid as one might hope (it’s based heavily on data from private companies and self-reported surveys, both of which can be flawed in many ways); which doesn't mean this isn't a real thing, but it doesn't mean it's a hardcore scientific fact, either.
It speaks volumes that this concept is featured in Dan Pink's newest work, When, as Pink is known for releasing interesting scientific works that are laden with "just so" stories: fascinating research, remarkable theories, but presented in formats that are sometimes light on contradictory evidence and anything that challenges the narrative and thesis of the book.
That said, like many concepts that seem real enough to be worthy of consideration but perhaps not as concrete as we might hope in the scientific sense (and to their credit, most researchers working on this topic will remind you this is the case), it can be a useful model for considering our own behaviors, especially when applied not just to how we think and act at certain ages, but at any moment we consider to be building up to a new milestone moment.
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