This is a very real mental trap that young adults frequently fall into - I certainly did! - and for the purposes of explaining how to avoid it, I think it's helpful to lay out in detail exactly how it can happen.
Let me explain:
As a child, every birthday is a milestone, and when you hit ten, you're finally in double digits! Huzzah! Eleven and twelve are ages of immense developmental significance, and then, at thirteen, you're Officially A Teenager. Fourteen matters because you're no longer a baby teenager; fifteen is frequently an age of cultural significance, and then there's sixteen, where you often start to gain more adult legal privileges. Seventeen is somewhat liminal, because it's an intermediary point between two responsibility-bestowing birthdays, but it matters for exactly that reason. Eighteen, of course, is a massive deal - the Age Of Technical Adulthood - followed by nineteen, the Last Year Of Your Teens. And then there's twenty, obviously, which means you're in a whole new decade, until finally, you get to 21, which in the US means you're now at the legal drinking age and elsewhere is celebrated as the last, you're-finally-out-of-childhood birthday.
In other words: up until 21, pretty much every birthday constitutes a tangible social level-up that's as perceptible to others as it is to you, and because most teenagers these days occupy extremely age-stratified school and social systems, there's a clear demarcation between those to whom the age-stratification applies (teens) and those to whom it doesn't (adults). Even in those last couple of years, when you're going off to university or entering the workforce or both, there's usually still a residual sense of novelty, where any adults you encounter in adult spaces will recognise you as a newbie, teachers and classmates alike will classify you according to what year of your undergraduate degree you're in (first year/freshman, second year/sophomore, third year/senior), and socially, if you're in the US, you're still marked out in certain contexts by not yet being of legal drinking age (regardless of whether you want to drink or not).
And then you turn 22, and it all just... stops.
There's no new level-up at 22. If you do an undergraduate degree or begin an apprenticeship starting at 18, as is common, you'll most likely be done by 22, but whether you continue with further study, go into the workforce full time or take a break, outside of a handful of hyper-competitive, youth-dominated fields and extended traineeships, there's no more layers of age-stratification in which to situate you relative to everyone else. Abruptly, you're on the adult side of the teen/adult calculus, but of course that younger era still feels more relatable, more you - that's what you're most experienced at!
And that's always going to be a difficult transition to make! But particularly for people who've never really socialised outside their own age bracket - who've never had a community or extended family experience that's routinely put them alongside a wide range of people of different ages before this point - the idea that they're suddenly not only able, but expected, to socialise with colleagues, cohort-mates, acquaintances, friends-of-friends, friends' partners, coworkers and other random people who could be as many as five, ten, fifteen, twenty, even thirty-plus years their senior? Suddenly, a lot of them have no idea what to do, even if they've previously been successfully social people, because those age-stratified systems they're used to, combined with the collective milestone-marking of youth, have always worked as built-in icebreakers.
By which I mean: prior to 22, when you meet new people in your approximate age bracket, you can ask, What grade are you in? or What school do you go to? or When will you graduate? or What's your major?, and immediately be assured that, whatever the answer, you've at least asked a comprehensible question anchored in a degree of shared experience. And at the same time, you're constantly surrounded by people and organisations who are actively working to create social opportunities for you, whether in the form of parents driving you to extracurriculars, classmates throwing birthday parties, schools hosting regular events or universities offering clubs and meetings and mixers. But removed from those frameworks, suddenly, everything's a crapshoot. And to make matters worse, you might not yet have that much practice at ballpark-figuring someone's age, which can make it even harder to know what sort of conversational gambit to go with.
(Which is one of the many reasons why, to go on a brief tangent, people who were deemed weird kids in high school often socially flourish in their twenties: because their perceived "weirdness" at the time frequently comes from not fitting in to a highly structured, age-segregated social hierarchy. Placed in more diverse settings that put a greater emphasis on things like shared interests, professional competence and social flexibility than on relative age combined with participation in collective milestones, however, and suddenly they've got more strengths to draw on than those whose social go-tos were built on scripts that abruptly no longer apply. But I digress.)
The point being, 22 can be wildly confusing in ways that all your prior experience with getting older really doesn't prepare you for. Which is where and why - to finally return to the specific point OP was talking about - the temptation to continue to associate oneself with youth and younger people can suddenly arise. You feel like you've got more in common with them; you understand the rules better; and really, aren't you closer to them than to someone who's 28 or 35 or 50? Well, sure; but you're also not them, and more to the point, the developmental aim now is to move away from them. That doesn't mean you can't still have younger friends or feel nostalgic about what you've outgrown, but you do have to acknowledge that the whole point of what came before was to prepare you, once you reached this point, to transcend it. To grow up.
And part of that is realising that adulthood is a journey you're on, not a destination you're already meant to have reached. In metaphoric terms, what you arrive at on turning 21 is the station: adulthood is the train you then board, and ideally, it'll keep travelling onwards for sixty-plus more years, and throughout the trip, you'll be talking to passengers who all got on at different times to you. Adulthood doesn't mean waking up one morning and finding that your love of fantasy novels has been replaced with a desire to file early taxes; it means you get fantasy novels and taxes, and the specific balance of freedom and responsibility that means a failure to do the latter due to preoccupation with the former (or with anything else) is entirely on your own head.
Which also means acknowledging that, however knowledgeable you were or felt yourself to be on finishing high school or college, you've ideally still got sixty-plus more years of learning to do about everything from the human condition to history to politics to how to cope with the Sisyphean task of laundry and dishes. You're meant to keep growing, in other words - and you can't do that if you mentally arrest yourself as Still A Little Baby Minor to try and escape your uneasiness with the fact that certain conversations are, in reality, far more complex than you previously realised. It's okay to admit you don't know something, or don't know enough; outside of work, there's no-one grading you on your ability to pretend you did the reading, and no penalty for shutting the fuck up if what's being said goes over your head, or for needing to take a day or two to properly digest it and form your own thoughts.
Because the thing is, being 22 is a finite state. Regardless of whether you feel fully adult or not, you're still going to eventually wind up turning 23, and then 24, and so on until you're far enough removed from where you were that continuing to think of yourself as a minor becomes increasingly absurd. But if you've made no effort to grow in that time, to actually acclimate yourself to the realities of adulthood - even, or perhaps especially, to the more difficult and upsetting parts of adulthood - then what you're in danger of becoming is that most terrible of things, an adult who can't take responsibility for themselves. The kind of person who (for instance) complains about how hard their life is while making no meaningful effort at improving it, because they've never learned to view discomfort as anything but a personal attack, and thus cannot reliably distinguish between an injustice and an inconvenience; between someone pushing their boundaries and the necessary if not always enjoyable business of cooperative living.
In short: adulthood is something you continuously work to have, not something you passively obtain, and if your response to the inherent challenge and necessary discomfort of that work is to try and claim you've got no responsibility to perform it, then all you're doing is committing yourself to being juvenile and annoying. And that is far more cringe than any fifty-year-old enjoying cartoon shows will ever be.
