SPOILER ALERT: Do not read until you’ve seen season 3 of “Stranger Things,” which premiered July 4 on Netflix.

“Stranger Things” has been world-beatingly successful because of the kids at its center. And they’ve seemed, at times, to be the reason why its success might not be sustainable.

When the sci-fi series emerged in the summer of 2016 — instantly becoming a national trending topic and developing into Netflix’s defining zeitgeist hit — its tone was set not by top-billed Winona Ryder, talented and cleverly cast though she is. The ease with which the cast of child actors (among them Finn Wolfhard, Millie Bobby Brown, Gaten Matarazzo, Caleb McLaughlin, and Noah Schnapp, joined in season 2 by Sadie Sink) summoned a sense of genuine curiosity helped define the show’s agenda, telling a story whose complication comes through the power of imagination and an element beyond the human. All of the children, in the show’s early going, come by being genuinely good naturally, and have a kid’s willingness to believe the impossible; Brown’s Eleven, an outsider like the geeky boys she meets, folds into the crew with ease.

But children have a funny way of growing older — and that seemed doubly dangerous for a show whose success was premised on its stars’ adorability and on their point-of-view. Not merely would the “Stranger Things” kids, at some future time, be too old to carry off the pure, giddy innocence of season 1, but the show, as constructed, might not be built to bear anything more dangerous. The second season, which differed from the first in magnitude of spectacle but not in tone, deferred this concern without dealing with it. But given that new seasons emerge every 18 months or so, real-world contingencies seemed to force the issue for season 3.

Fans should not have worried. This new season is not the “Stranger Things” that came out in 2016, and that’s a good thing. The growing-up of its central characters has prompted changes in sensibility and in structure that refresh the show and give it something powerful to say. Growing up doesn’t have to be hard at all.

First and most obvious among the changes made this season are the divisions between the crew, with Dustin (Matarazzo) separated from his cohort in a way that happens easily enough; he goes to science camp and returns to find a friend group that’s moved on without him. His teaming with Steve (Joe Keery) and new character Robin (Maya Hawke, brilliantly giving wit and life to a character whose complications push the eighties-movie-reference frame of the show) to crack the case of mysterious Russian presence in town opens up the show’s universe in a natural way. Steve’s alliance with his younger cast members in season 2 was a smart choice that continues to lend him a bit of levity; in this combination, Dustin also gains a not-unwelcome bit of melancholy, a sense that time is moving him away from his friends sooner than he’d like and that he’s obliged to make the most of it.

Elsewhere, the fact of the characters’ increasing age is signaled with a literalism that can sometimes thud; Mike (Wolfhard) and Eleven are now a couple, with all that implies for young teens in the somewhat more innocent mid-1980s, and their first scene in a clinch is jarring, as if by design. (These are still relatively young people, and we’ve known them since they were a few years younger even than that.) But their inability to figure out quite how to cope with one another, with Mike missing every signal and Eleven’s powers amping up the tumult in her heart, feels familiar.

The characters aging up, coming into closer contact with their older peers and behaving a bit older themselves, closes off certain old possibilities. Will (Schnapp) is dispirited that his pals are no longer engaged by the possibility of playing D&D, that they’ve put childish things behind them before he was ready. But it opens up other ones, too: There’s a recognizable, poignant subtext to Will’s organizing the role-playing game as a “no girls allowed” event, made text for a glancing moment when Mike yells at him that he is only so dependent on childish things because he doesn’t like girls. The moment hangs in the air a beat too long, as it develops into a question: Does Mike mean he doesn’t yet, or that he won’t ever?

That remains to be seen in season 4 or beyond. And for the first time, I believe that “Stranger Things” could have a run that rivals “House of Cards'” six seasons or “Orange Is the New Black’s” seven. It’s not merely that Will receives a bit of complication. (Indeed, the show could benefit from more, not fewer, character beats: McLaughlin’s Lucas, in a future season, deserves an arc as complex as his castmates’). It’s that said complications inform every aspect of the show. This season takes on both the influence of the world outside Hawkins and an epidemic that makes people behave like monsters, strangers to themselves, one that verges far deeper into horror than previous seasons and makes use of the horror genre’s metaphorical power. It also dispenses with adult supervision entirely, sending the two major grown-up characters on a sojourn that makes no sense and adds little but that moves them off the board to make room for burgeoning independence.  And, in the story of Eleven’s powers, which are sometimes mighty beyond her ability to control them and sometimes gone, we see an individual at once overwhelmed by growing up and learning what it means to be herself without the crutches on which she’s relied previously. That she eventually uses those powers to uncover unresolved trauma within the personal history of Billy (Dacre Montgomery) helps bring the story home. Once, the monsters were just interdimensional creatures. Now, they’re humans, with all the complexity that implies.

All of which may sound like overthinking a show whose elemental appeal is the charisma and vibrancy of its ensemble cast. So it’s worth noting the ways in which the cast sells their journey, both in quiet moments and in sharply observed ones. The season contains several references to “Back to the Future,” and the young characters, especially as the season reaches a coda in which the Byers family and Eleven mourn Hopper (David Harbour) and prepare to move, confront the ways entering one’s own future is scary, and hard, and un-fun in ways I might not have suspected the show was capable of. It had never tested itself like this before — and it finds a way to convey growing up with the same featherweight touch it brought to stories of simply being young.

To wit: Mike’s season-ending declaration to Eleven that, though she is moving away, he hopes she will return for Christmas, so they can play with their new toys together, is movingly followed by the blush of embarrassment, the knowledge that together, they have more to do than imaginative play with Transformers even if that’s what he still would most enjoy. And Dustin’s phone conversation with his camp girlfriend Suzie bursts into joyous, strange life as the pair serenade each other with the “NeverEnding Story” theme before she provides a clue that helps save the world. It’s a blend that suits this story, one whose end I hope comes after we bring these characters into young adulthood: Dustin is engaging with a peer in a new way, a way that in a few years will subsume his social life and make it harder to have the sort of friendships on which this show is premised. But that engagement happens through an unself-conscious embrace of the fun of popular culture and of belief in the impossible, whether that impossible is defeating an undefeatable monster or keeping a relationship going at a great distance. Watching Dustin and his peers keep that belief in the face of getting older — watching them run from the possibility of becoming jaded as much as they run from the creatures haunting Hawkins — will give this show life for years to come.