I've had an idea in my head for a story (but have like 4 others I'm already working on) that was inspired by a friend's personal medical experiences.
Basically, the idea is thus: in seeking a cure for X disease, researchers discover the means to renew cells. They initially don't release it to the public, because they're trying to perfect it. The goal is to sort of hit the reset button on, say, breast cancer. Try as they might, however, they can't seem to be able to narrow the treatment's focus to a particular spot on the body. In order to get rid of the tumor, they've got to apply the treatment holistically or not at all. The side effects are met with some concern from investors and funding slows.
But like some other treatments that didn't exactly work out as hoped, the side effect becomes a feature after a less idealistic entity scoops up the patent.
The rich and famous quietly begin using the treatment to get their peak looks back. The word doesn't spread all that quickly because the powerful see it as just another way to hold their advantage over the plebes.
But what even the rich don't know, because the doctors haven't told them, is that there really isn't a limit to the treatment. While everyone is using it to maintain their adult beauty, it's capable of going further.
And after word gets out about the treatment and it becomes more commonplace, some begin to experiment with this.
For whatever reason, memory remains intact. So no matter what happens, the individual remembers their entire life.
It gets abused by governments. "Unreformable" convicts are forcibly sent back to infancy and given up for adoption in the hopes that nurture will win out over nature.
It becomes so popular in Japan for women to go back to a very young age (think Major Kusanagi at the end of the first movie) that new laws have to be passed to sort things out.
Eventually this process is so commonplace globally that much like sexual or gender identity, people begin to identify with certain ages. The typical recommendation is that people make it to 40 before they make any decisions about their "true" age.
Turns out, a lot of people decide they're teenagers after all. Outside of certain cultures like Japan, not too many people opt for anything less than that.
Because while memories remain intact, there are some drawbacks. For one thing, going so young that you are no longer an adult height makes things like driving and other adult activities harder. For another, due to the difficulty in differentiating between a first-time child and a second-timer outside of a doctor's office, drugs and alcohol are still a no-no.
Even being a teen means having to deal with some of the hormonal stuff. But having life experience does make that a little easier.
There are a few other side effects. Second-timers are a little bit physically weaker than they were the first go round. This can exacerbate any medical issues one might have had at a certain age if one returns to that age.
My story was going to focus on a woman who went back to around 19 because she loved being a young, dumb party animal college girl, but found out that the aforementioned side effect meant she started to suffer from the bedwetting that the had previously kicked just before high school. And it all goes downhill from there.