Jak'tahla is a Klingon word that roughly translates into the Klingon version of puberty that affects Klingons during their teenage years. It is characterized by accelerated hair growth, increased aggressive tendencies, the appearance of gorches on the Klingon's face, among other things.
In 2375, the metaphasic radiation from the Ba'ku planet's ring's caused Lieutenant Commander Worf to re-enter jak'tahla. (Star Trek: Insurrection)