The title of Meg Jay's Defining Decade: Why Your Twenties Matter-And How to Make the Most of Them Now is pretty self-explanatory: your 20s matter, don't waste them. That's a jarring idea in a culture that celebrates "extended adolescence", gap years, and parental insurance coverage until you're 26.

But increasingly, I think Meg has the right idea. The reason is that everything takes so damned long. Want to go to college? That's a minimum of 4 years and as much as 9-10 for medical school, or a Ph.D. Want to get married? That's at least 3-4 years: meet the person, decide you like them enough, engagement, wedding planning, etc. Same deal for buying a house, moving cities, or making progress in a career.

And then there's kids, if you ever want those, you'd better have all this done by the time you hit your mid-30s, otherwise you're going to be writing a dissertation, or starting a new job, right as the kid is waking you up multiple times each night. And that's best-case, assuming fertility issues—the hidden price of the Bay Area's astounding wealth and innovation—don't become a problem. Getting the sequencing of all this right, especially with two people, takes a lot of work and it's probably only going to get worse, as workplaces demand more training and specialization with each passing year.

All to say, don't waste time. Whatever your ambition, whether to travel, open a restaurant, have a family, write a book: start today. Get after it.