The Age of Decision was a stage in a Talarian adolescent's life.
When a boy reached the age of fourteen, he underwent a ceremony of initiation, and after that had the freedom to make his own choices. In 2367, Endar asked Jono whether he wanted to return to his grandmother Connaught Rossa, saying because he had reached the age of decision, he could choose. Jono chose to stay with the Talarians. (TNG: "Suddenly Human")