Adolescence or youth was a period within an organism's life span after childhood, in which they were starting to mature but were not yet an adult, such as a teenager.
In Humans, adolescence began at around twelve years of age and ended at around eighteen years of age. During adolescence, Humans and Bajorans gained additional DNA patterns that they kept in their adult life. On Stardate 46235.7, Jean-Luc Picard, Keiko O'Brien, Ro Laren, and Guinan lost these DNA patterns, rendering them as children who appeared to be twelve years old but they eventually got them back by running copies of their adult DNA patterns through a transporter. (TNG: "Rascals")
As a teenager, Mira Romaine suffered from psychosomatic illnesses occasionally, but nor more so than was normal for her age group. (TOS: "The Lights of Zetar")
Beverly Crusher believed that Human adolescents should be misbehaving and dating, and so was concerned about her son Wesley Crusher's well being when he did not misbehave and only dated a few times. Picard told her that Wesley could do whatever he felt like and that there was no one way to be an adolescent, but also noted that he himself got into a lot more "trouble" than Wesley did when he was seventeen. Later, when Wesley was seen asking a girl out, Beverly noted that he was doing what boys his age "should" do. (TNG: "Evolution")
Data once noted that in the past, automobiles were involved in teenage mating rituals. (TNG: "The Big Goodbye")
In most Betazoids, their telepathic gifts developed at adolescence, however, occasionally a child was born with their telepathic abilities switched on. Most Betazoids born that way never led a normal life as the noise of other peoples' thoughts and feelings were overwhelming, incomprehensible, and painful, especially to a child. (TNG: "Tin Man")
Jake Sisko spent the majority of his adolescence on Deep Space 9 (DS9: "Emissary", and later episodes), while Wesley Crusher spent his late adolescence on board the USS Enterprise-D (TNG: "Encounter at Farpoint", and later episodes) before joining the academy. (TNG: "Final Mission")
In Klingons, the Age of Ascension took place during a Klingon's adolescence and it was the time that the Klingon became a warrior. (TNG: "Firstborn", "The Icarus Factor", "Sins of The Father"). In Ferengi, it took place once adolescence had ended and was the age at which the Ferengi could leave home and earn their own profit. (DS9: "Family Business")
Mardah did not recognize the diversity among teenage boys until she met Jake Sisko. (DS9: "The Abandoned")
Tom Paris allegedly lost his virginity during his late adolescence, at age seventeen, in his bedroom while his parents were away for the weekend. (VOY: "Threshold") A year earlier, he had taken his father's shuttlecraft out for a joy ride but crashed it in a lake. (VOY: "Vis à Vis")
Charles Evans was taken from his home and given telepathy, psychokinesis and other such abilities at age three by the Thasians and lived on Thasus until James T. Kirk and his crew discovered him, at which he was a healthy boy in his late adolescence. (TOS: "Charlie X")
In their adolescence, Kazon boys were required to kill another humanoid to earn their Kazon-Ogla name. Failure to do so was seen as cowardly. (VOY: "Initiations")
Kissing while on duty was considered socially acceptable among adolescents, but was considered juvenile in adults. (VOY: "Scientific Method")
When a pathogen infected a Borg cube, it killed all the adults on board, but two adolescents, three children, and one infant survived due to having been in maturation chambers at the time. When the maturation chambers malfunctioned, these individuals attempted to take charge, but were behaving self-centered. Tuvok believed that a lot of adolescents behaved this way. (VOY: "Collective")
In his adolescence, Tuvok once felt romantically attracted to his classmate Jara, which got him in trouble, and he did not want to suppress his emotions. (VOY: "Gravity")
A four-year-old member of the Q Continuum was considered an adolescent. (VOY: "Q2")
In Arret, James T. Kirk and his crew temporarily transformed into adolescents due to time running backwards. (TAS: "The Counter-Clock Incident")
Benjamin Sisko had bad memories of his adolescence so was unsure how to raise an adolescent. (DS9: "The Nagus")
When Miles O'Brien was seventeen, his father signed him up for a music academy he did not want to attend, so Miles joined Starfleet instead. (DS9: "Shadowplay")
Jem'Hadar reached adolescence in a matter of days. (DS9: "The Abandoned")