Adolescence Syndrome, also known as Puberty Syndrome (思春期症候群, Shishunki shōkōgun) is a term used to describe abnormal experiences during adolescence as a result of sensitivity and instability. Although considered an urban legend, this occurrence is experienced by each of the main characters.
Those who have Adolescence Syndrome are susceptible to various events or experiences that would be considered abnormal and irregular. These effects happen whenever the individual goes through a distressing or agonizing event, such as Kaede Azusagawa being bullied online or Mai Sakurajima being overwhelmed by show business.
Mai's effect from Adolescence Syndrome is that she became invisible and that her existence had been subconsciously forgotten by nearly everybody. This stemmed from a previous wish to be forgotten by the world as a result of being forced to do a swim-suit photoshoot by her mother, and Mai realized that her mother was just using Mai's success for money.
This effect was subdued once Sakuta publicly confessed his love for Mai in front of the school; effectively forcing the students and teachers to acknowledge her existence.
This occurrence is much like the concept of Schrödinger's cat; unless one opened the box to check on the cat's existence, their consciousness will not acknowledge the existence of the cat at all. Thus, if people do not acknowledge the existence of Mai, they simply will not perceive her existence to their consciousness.
Kaede is susceptible to spontaneous physical injury if she constantly receives slander, hate speech, or defamation through social media; the hatred in those words will transfer into physical injuries. These injuries can range from a mild paper-cut to a full-sized scar and a bruise.
Along with receiving injuries, Kaede experienced amnesia as a way to sever herself from her traumatic experience, gaining a new personality as a result.
The known counter-measure to receiving injuries is to isolate oneself from social media and the internet as a whole, which is why Sakuta threw his phone in the ocean for Kaede's sake. As for her amnesia, it was subdued once Kaede completed her list of goals to be more social. She regained her previous personality and memories, but lost the ones she had as an amnesiac.
Much like his sister, Sakuta is also susceptible to sudden injury; in his case, he suffered three deep cuts to his chest a day after witnessing Kaede suffer from Adolescence Syndrome herself. These injuries were severe enough to send him to the hospital.
Once Kaede reverted to her previous self, Sakuta's wounds reopened as he grieved the loss of Kaede's recent memories. It is unknown specificially what his adolescence syndrome is caused by, but it can be noted that Sakuta's wounds appeared and reopened in times of great emotional distress (such as Kaede's adolescence syndrome occuring).
When Tomoe is unable to accept her current circumstances, she is capable of eliciting the future from an earlier point in time (a concept based on Laplace's Demon) and is able to rewind and repeat the current day in the simulated future. Once time is rewound in the simulated future, the only people who can perceive the time loop are Tomoe and Sakuta. All other events would occur exactly like they previously did, provided that Tomoe and Sakuta don't alter them.
This effect was subdued once Tomoe admitted that to Sakuta that she loved him, while accepting that his heart was with Mai. Time then rewound and started before the simulation was ever elicited.
Rio's feminine body parts developed earlier than her classmates, and some boys began ogling her. Because of this she started to hate her body. In an attempt to get attention, Rio began posting suggestive photos of herself online and kept continuing despite the fact that she didn't like it. This contradicting method and outcome split Rio into two versions of herself; one who wanted attention and another who didn't like how her body was used to get attention.
This effect was negated once the Rio who wanted attention realized she wouldn't lose the attention of her friends and after the remaining Rio found the courage to ask her other self to attend the fireworks show, thus recombining the two selves.
Nodoka's effect from Adolescence Syndrome was that she had switched bodies with Mai. This stemmed from her inferiority complex towards her sister, which had lasted since her mother began comparing her success to Mai's.
This effect was nullified after Mai told Nodoka to not be like her, but be the best version of herself. After Nodoka came to this realization, both Nodoka and Mai reverted back into their original appearances.