Time is a funny thing. It sneaks up on us when we're not paying attention, and keeps on ticking even when we do. As young children and adolescents, we aren't yet old enough to consider the bigger picture of how precious and sad and sometimes scary and other times joyous and entirely fleeting our very youth is. As adults with imbued perspective and a bevy of additional experiences under our belts, we are more conscious of the process of life, yet still, year after year, cannot quite believe how quickly it passes us by. "Boyhood" is about all of this and, basically, everything. Chronologically shooting for thirty-nine days over an unheard-of span of twelve years, writer-director Richard Linklater (2013's "Before Midnight") charts with stunning, uncanny verisimilitude the growth of a little boy of six who transforms over the course of a breathtaking 166 minutes into an 18-year-old man. This by itself is startlingly powerful and joltingly cathartic, yet it is Linklater's and his brilliant actors' handling of their all-encompassing material that lifts the film to a level which delves far deeper and becomes more significant than what viewers will be anticipating.

Mason (Ellar Coltrane) is a daydreaming first-grader when we meet him, waiting to be picked up from school while staring off into the clouds. A child of divorced parents, he nonetheless has a reasonably happy home life with struggling but dedicated single mother Olivia (Patricia Arquette) and year-older sister Samantha (Lorelai Linklater). Their dad, Mason, Sr. (Ethan Hawke), has been gone for a while—they have been told he's been working in Alaska—but now he has returned to their home state of Texas and is hoping to be a more consistent figure in his kids' lives. Tired of barely making ends meet, Olivia packs up and moves to Houston with Mason and Samantha as she works on getting a college degree. As Mason ages through several more relocations, different schools, shifting influences and new stepparents, he morphs before the viewer's eyes into a thoughtful teenage boy of quiet introspection and artistic talent, one who is still feeling his way around the whole "growing up" thing. What awaits him on the other side comes with its own set of pressures and responsibilities, something that his forever-evolving parents know firsthand; although they are in their forties by picture's end, they, like Mason and Samantha, are works in progress.

It is nothing short of stunning to watch "Boyhood" unfold, a seamless, expansive assemblage of human experiences and memories cohesively condensed down to under three hours. Not merely the story of a boy coming of age—though rest assured that this ranks alongside 1959's "The 400 Blows," 1985's "Smooth Talk," and 1986's "Stand by Me" as a new apex of the "coming-of-age" subgenre—the film just as emphatically encapsulates what it is like to be a sibling, a parent, a teenager and a grown-up. Working from a loose, semi-improvised script that was developed gradually and informed by his younger actors' maturation, Richard Linklater has somehow cosmically keyed directly into the beauty and the heartbreak of life itself—and all the notable large and small moments in between. There are ball games and bowling, notes passed in class, the resentment of having to move, the awkward feeling of facing classmates after a bad haircut, the thrill of attending a long-anticipated event (in this case, the 2005 midnight book-release party for "Harry Potter and the Half-Blood Prince"), the fear that goes along with being trapped in the middle of unhealthy adult relationships, and the pubescent excitement of talking to the opposite sex. There are late-night conversations between a father and a son (a scene shot years ago where they discuss the possibility of future "Star Wars" features now plays like amusing prophecy). There is the exploration of religion, and a parent's political passion as a Presidential election approaches (Mason, Sr. sends his kids out to campaign for 2008 hopeful Barack Obama). There's experimentation with drugs and alcohol, and the "sex talk," and first loves, and getting The Beatles' "The Black Album" as a 16th-birthday gift. There are bosses, and teachers, and grandparents, and art shows, and college tours, and graduation parties, and actions that have consequences years down the road, and more—so much more—where all of that comes from.

Because the actors are living with their characters and aging along with them for over a decade, artifice is nonexistent in their performances. To see their familial relationships build and intermingle through the years is astonishing in its truthfulness. Olivia didn't plan to raise two kids on her own, but that only drives her to want to be the best parent she can. By working toward a university teaching career, she hopes to better provide for them while setting a positive example. Her luck with men, however, is not as fortuitous, as she and Mason and Samantha will find out. Mason, Sr. is, thankfully, not treated as a deadbeat dad; he genuinely loves his children and wants to be there for them, but he must follow the custody agreement and eventually starts a new family. Olivia and Mason, Sr. are parents who do not always get along but come to reveal a reciprocal respect for each other—one that unveils itself gradually and naturally. Meanwhile, friends, lovers and acquaintances move in and out of their orbit, as so many people do in our own.

Ellar Coltrane emanates an increasing sensitivity and soulfulness as Mason, the imagination and precociousness of his 6-year-old self paving the way for the low-key, contemplative aspiring artist he becomes. His work is exquisite at any age, not once exhibiting the affectations or self-confidence of a child of Hollywood. That he is a normal kid, born and raised in Texas, is crucial to the story's semi-vérité reality. The same can be said of Lorelai Linklater (Richard's daughter), who has a great look and an even greater innate appeal. Seeing her Samantha go from a bouncy, animated little girl who cheerfully annoys her brother with a full song performance of Britney Spears' "Oops!...I Did It Again" to a quieter, more withdrawn teenager feels precisely right to the metamorphosis that occurs when one's innocence starts to fade.

As their father, Ethan Hawke (2013's "The Purge") is excellent, his sporadic appearances becoming more frequent as he takes control of his responsibilities and starts to settle down. From a struggling musician to someone who has made peace with the unexpected, disappointing, but finally miraculous ways things have turned out, Hawke's character arc is one that stands along the periphery but is not less affecting because of this. Perhaps the most deserving triumph among the ensemble is Patricia Arquette (2006's "Fast Food Nation") who, save for her sterling, Emmy-nominated work on TV's "Medium," hasn't had a role this juicy and copacetic in at least fifteen years. Arquette puts her everything into mother Olivia, a part that builds in complexity and poignancy as she earns a satisfying job, faces two failed marriages, and can't quite believe it when it is time for Mason to leave to college. "I just thought there would be more," she tearfully says in a rush of uncontrolled emotion. When we look back on the past stages of our lives that we will never—and can never—get back, isn't this notion always at the heart of what we feel?

Richard Linklater has spearheaded a number of inarguable modern classics, from 1993's nostalgic, '70s-set slice-of-life "Dazed and Confused" to the marvelously romantic "Before Sunrise"/"Before Sunset"/"Before Midnight" trilogy starring Ethan Hawke and Julie Delpy, but in terms of its epic yet intimate scope, pure ambition and unfaltering, unforced storytelling prowess, "Boyhood" could very well be his masterpiece. Shot—and, by extension, set—between early 2002 and 2013, the movie serves as a loving time capsule in addition to an absolutely timeless snapshot of the pangs and transcendence of human nature and existence. It is mesmerizing, even awe-inspiring, to behold, complemented by a collection of impeccably chosen soundtrack cues by Coldplay, The Hives, Sheryl Crow, The Flaming Lips, Paul McCartney & Wings, Bob Dylan, The Black Keys, Cobra Starship and, most unforgettably, Family of the Year. Indeed, Linklater's magnum opus offers more than what one has any right to expect from a film while simultaneously illuminating the incalculably infinite boundaries to which cinema is capable of going. When "Boyhood" comes to its wistful, wonderful, note-perfect conclusion, the experience of having watched these people onscreen has become the experience of watching the tale of our own lives.