Genpuku, literally "first clothes," was a coming of age ceremony used to mark the entry of young men into adulthood for hundreds of years in premodern and early modern Japan.
The ceremony was held at some point between the ages of 12 and 16, varying by region and time period. The boy would be taken to a Shinto shrine, where he would be presented with his first set of adult clothes (hence the name of the ceremony), would have his child's hairstyle changed into that of an adult, and would be given a new, adult given name to replace his childhood name.
A very similar ceremony, called Mogi (裳着) was sometimes performed for girls, in which they would similarly be given new, adult clothes.
Originally such ceremonies were only practiced by the nobility, but the practice gradually spread to the warrior class, and eventually even to wealthy commoners.
Today the Genpuku and Mogi ceremonies have been replaced by the Seijin Shiki ceremony which takes place every year on the second Monday of January for all men and women who turn 20 years old that year - 20 years old being the the present-day age of legal majority in Japan.