Once upon a time — long before everyone knew that millennials are a bunch of Instagramming snowflakes who demand trophies for everything — Generation Xers were a bunch of apathetic slackers. Before that, boomers were the self-absorbed “me generation.” Decades earlier, Silent Generation members were accused of being cautious and conformist.

This is just the way it goes: The old heap scorn onto the young, forgetting that they themselves were once scorned. But life always goes on, and the kids always come to power (and complain about the next generation). And right now, a small but growing group of millennials have fought their way in.

In “The Ones We’ve Been Waiting For,” Time national correspondent (and millennial) Charlotte Alter takes us on an early tour of millennial political leadership. The book opens with a dramatis personae of sorts — a list of 10 millennials who have had early political success, all of whom Alter has interviewed. They range from megawatt stars (Pete Buttigieg and Alexandria Ocasio-Cortez) to officials without national profiles (Rep. Haley Stevens of Michigan and Ithaca, N.Y., Mayor Svante Myrick).

Alter uses their stories to explore the events that have shaped millennials: 9/11, wars in Iraq and Afghanistan, Barack Obama’s election, the Great Recession, Occupy Wall Street, school shootings, Black Lives Matter, steadily rising student debt, and the election of Donald Trump.

Which is to say: Millennials have already been through a lot.

It’s to Alter’s credit that she doesn’t try to squish this all under one overarching thesis; “The Ones We’ve Been Waiting For” is an exploration, not a treatise. “This book is about what they believe and why they believe it, and what America might look like when they’re in charge,” she writes in her introduction.

But she’s nevertheless attempting a massive undertaking: covering a list of generation-shaping news events, any one of which could merit a book of its own, with 10 central characters, via multiple storytelling formats. There are 10,000-foot overviews of cultural forces, segments spent tightly at the side of one of her millennial protagonists and brief interludes about particular, disastrous events — shootings and hurricanes.

All of which means that “The Ones We’ve Been Waiting For” often feels like it’s trying to take on too much.

The book is at its best when it tells stories through the eyes of Alter’s millennial politicians. In one chapter, she examines the wars in Iraq and Afghanistan through the experiences of Buttigieg, Rep. Dan Crenshaw of Texas and Rep. Max Rose of New York — all of them veterans, all of them millennials.

It’s not a complete story of how millennial veterans experienced those wars — none of these three characters are women or people of color, for example — but then, it’s not quite trying to be. Instead, it’s a particular story. It’s a story of how these wars shaped these three politicians, and how they might shape the country because of it.

These stories are all buoyed by Alter’s sharp writing and engaging voice. She is by turns sarcastic, funny and sincere, but always conversational; you get the sense that she has absorbed and is mirroring back to you how her characters talk (“Max doesn’t like it when you ask him about what the explosion felt like, because it was an explosion — what do you think it felt like?”).

And it’s in exploring these politicians that she comes to her sharpest observations. After examining Buttigieg and Ocasio-Cortez’s respective roles in the Democratic Party, Alter concludes: “They wouldn’t like to admit it, but their goals were largely aligned: universal health care, a massive government investment in addressing climate change, a twenty-first-century social safety net, and a reformed democracy.”

But when Alter strays from the particular, she loses some of that sharpness. Often, she leaves her cast of millennials to summarize major events or phenomena. In the process, she makes broad statements that whisk by before you can digest what she’s saying. “Obama seemed so powerful, so transformative, that many of his young supporters assumed he had some kind of magic wand that could miraculously fix the problems he described,” she writes. “They put him into the White House: fixing the country was his problem now.”

Or, in a chapter about baby boomer parenting: “The boomers — the ultimate individualists — didn’t think much about the next generation in a broad, political sense, but they thought a lot about the next generation in a specific, individual sense. Boomer political calculations were often rooted in where their sons would go to college, whether their daughters would be able to play soccer, all the triumphs and failures of their own individual progeny.”

These generalizations feel like they could be true. But how true are they? Does Alter have data to back up these ideas? And indeed, popular generalizations about millennials can be wrong. At one point, Alter cites millennials’ reputation for job-hopping, when data shows that they are not more prone to change jobs than older generations were.

It is true that data may not resolve some questions Alter raises and that an author does get some room to observe and speculate. But at times, Alter’s generalizations cry out for further evidence.

In addition to personal stories and historical recaps, Alter has yet another storytelling mode: interspersed between the chapters are brief, newsy rundowns of events that shook America while millennials came of age — Hurricane Katrina, Hurricane Sandy and several mass shootings. Some of these recaps are composed from tweets and headlines, rather than narrative prose.

Alter writes in her author’s note that these mini-chapters are meant to mimic how “the last twenty years have been punctuated” by these traumatic events. In other words, they’re meant to be jarring. But the effect is a string of interruptions that illuminate little.

Still, Alter’s book will come as a tonic to millennials who have grown weary of boomers’ well-worn complaints about them. That’s because “The Ones We’ve Been Waiting For” takes millennials seriously — and likewise takes seriously older generations’ responsibility for millennial woes, such as economic insecurity.

To Alter, social media and complaints about student debt aren’t symptoms of an infantilized generation. They are simply facts of millennial life and are entering political office along with all the new millennials who ascend to power — boomers’ complaints be damned.

The Ones We've Been Waiting For

How a New Generation of Leaders Will Transform America

By Charlotte Alter

Viking. 344 pp. $27