A confluence of events provided the factors for the paradigm shift. 

In the 80s the media went wild for 2 EXTREMELY RARE cases of child abduction by a stranger. Adam Walsh and Etan Patz (a while later Jacob Wetterling). Kids were on milk cartons. TV news got much better ratings when they covered scary stories, involving blood and death. Heart-wrenching stories with adorable blond 6 year old victims sold like hotcakes. So Stranger Danger Panic emerged.

TV personalities got rich and famous by terrifying the population even more. Their shtick required that the victim be innocent and vulnerable. So they constantly used words like "defenseless victim", "impressionable minor", "naive child", and "inexperienced teen"

Big sexual abuse lawsuits against the Boy Scouts and the Catholic Church popularized the idea that child sex abuse is extremely harmful and extremely common. Neither of those things is true. Well-meaning people conflated the concepts of minor and child so they could inflate statistics. Teens can, of course, consent to sex, they've been doing it for all of human history, but admitting that would mean that you couldn't prosecute or sue for millions of $$$. So well-meaning but teen-phobic sex-phobic Puritans invented the mantra "minors can't consent". They have to repeat it constantly, because it is obviously not true. 

The Stranger Danger Panic became Sex Predator Hysteria. All throughout history people in their late teens dated and sometimes married people in their early twenties. Romantic relationships between 16 year olds and 24 year olds was always common. Also completely harmless. But nobody was thinking rationally. Everybody was terrified. They had convinced themselves that such a romance would create terrible trauma. They openly called it child rape. 

but something even worse was happening that they had to deal with. Many of the younger partners in these completely normal couples said openly "I consented. Everything was fine." If the fear-mongers accepted this as true their whole worldview collapses, so they expanded the concept of "grooming". Their new interpretation was that it's a kind of brainwashing. The abuser is nice to the victim, and the victim thinks he or she likes it, but total strangers know better and can see that it isn't two young adults making love but in fact child rape. This requires the "impressionable minor" or the "inexperienced teen" to be too stupid to see that he/she is being abused. To consider normal young adults aged 16 or 17 "defenseless victims" or "naive children" you need to believe that their brains are deficient. So the public was hungry for any way to argue that young adults are incapable of adult thinking. That was the fertile ground the junk fMRI studies landed in. 

Then we got #MeToo. Just like Stranger Danger a few extreme cases got talked about so much that everyone thought it was everywhere. Harvey Weinstein is a totally vile piece of shit rapist. He used his power to coerce. But since Sex Predator Hysteria was well established everyone was super eager to declare that anyone who has more power than their lover is in fact raping that poor victim. It was completely ridiculous, but nobody cared. Nobody wanted to think. They wanted fear and anger. That's the nature of hysteria. and it fit beautifully into the infantilization paradigm. If you're 19, a vulnerable teen, obviously your 26 year old girlfriend has more power than you, so you cannot consent, so you are being sexually abused. and self-righteous "saviors" who have never met you or your girlfriend can feel superior and morally pure. 

When I was 17 in the 1980s my boyfriend was 23. It was not a big deal. Nobody even mentioned the age gap between us. There was nothing out of the ordinary about it. By the time #MeToo came around I hadn't thought about him for decades. but when I shared the facts of our relationship on Twitter in 2020, tons of people were eager to tell me how I was an idiot who got groomed and abused. I was not a child. I was not raped. I know. I was there. but social media loves spreading the infantilization narrative. That way they can create salacious stories of sexual abuse when the true story is a rather boring "two young adults did a romance"