Another way to see how child-like popular culture has become is to look at portrayals of romance, courtship, and so on.
Adults might have all sorts of romantic encounters and relationships -- with a younger, older, or same-age partner, for a short or long duration, with playful or serious intentions, and where one is playing hard to get, where both are, or where both are moving straight for the goal line. Sex might be scary, awkward, and shameful, or it might be comforting, natural, and feels-so-right. Break-ups may be petty, spiteful, and vindictive, or they may be friendly, or fading-out, or abrupt.
From the juvenile point-of-view, however, it can only be two soulmates who were created for each other, who the gods and mankind alike are conspiring to keep them apart, and who will either triumph in immortal romantic glory or else be ruined into oblivion. You stop thinking that way once you've had some experience in the dating-and-mating arena, hence holding such views is a sign of naivete vs. experience.
There is another juvenile view, of course, which is that the opposite sex may be OK, but is not really to be trusted enough to open up to and get involved with in any real sense. Social avoidance combines with self-focus to lead one or both to only value validation and ego-inflation, while stringing the other one along. It could be in a bratty "I'm the shit" way, or it could be twee / emo indulgence in awkwardness.
Grown-up portrayals of romance will feel natural and allow the audience to go along with them, while juvenile portrayals will feel forced and take the audience out of the moment (assuming that the viewers are not children themselves). One is generous, the other is self-indulgent. This naive vs. naturalistic difference is more important than merely what age the characters are.
With that understanding, let's take a look at how film romances have changed since as recently as the 1980s. I'm leaving rom-coms out of this since I already wrote two posts (here and here) on how romantic comedies devolved into chick flicks, with their princessy demands for courtship utopia. I'm also leaving aside the kiddie Disney / Pixar / Etc. movies, since those go without saying. But the fact that so many of the most popular movies are from that mold only makes my point stronger. I'm mostly sticking with movies from the top 10 to 20 at the box office, to avoid cherry-picking.
Anakin Skywalker and Padme Amidala -- forced, fake, juvenile. (Ditto for Arwen and Aragorn -- forced, emo histrionics about "I need you here or I'll diiieeee!") Han Solo and Princess Leia -- natural, genuine, mature.
The guy and girl from Titanic -- forced, emo, juvenile. Michael Douglas and Kathleen Turner in Romancing the Stone -- natural, mature, grudging acceptance leading to playfully teasing enjoyment.
Neo and Trinity from The Matrix series -- over-the-top emo fantasy writing by goth rejects on the bus ride to middle school. Deckard and robo-Rachel from Blade Runner -- not the most life-like relationship, yet still believable as one of those shouldn't-do-it affairs that you get into out of attraction only, fight with each other, and somehow make amends later on.
The blue alien chick and the Earth dude in Avatar -- forced, emo. Harrison Ford and the Amish lady in Witness -- sincere, both their halting and advancing phases are motivated and natural.
Spiderman and Mary Jane -- forced, kissing in the rain with a mask on = the ultimate emo cliche fantasy. Superman and Lois Lane (in the Richard Donner movies) -- he has natural charisma, and she has natural damsel-in-distress appeal, although she pretends that she does not, and that lets Superman feel extra cocky for getting to save a gal who thinks she can take care of herself.
Bella and Edward -- emo, the girl just wants validation rather than a connection, a blue-balling brat. Daniel and Ali (with an "i") from The Karate Kid -- both begin sheepish but learn to trust and look out for one another, and both want a real connection rather than mere validation-from-a-distance.
That's more or less the range of types shown today (aside from the chick-flick types already discussed in the other two posts). That's not even mentioning Peter Venkman and Dana Barrett in Ghostbusters, Jeffrey and Dorothy Valens in Blue Velvet, Indiana Jones and his ex-ex-girlfriend Marion, and Tom Hanks and his love interest in Big.
The last example is memorable because it shows that maturity is about relating pro-socially toward others, not occupying a certain rung on the status ladder. Tom Hanks' rival may have joined the rat race, climbed his way up to management, bought a car and an apartment of his own, and cohabited with his significant other -- but he is petty, insecure, and cowardly. He is a small man, who can only lash out and not connect with others. If the movie were set in the 21st century, he would waste time whining about and taking empty potshots at his rival on Facebook. Tom Hanks' character is still growing up, but he's not childish.