youth: the early stretch of a life, from childhood into young adulthood, seen as a whole season
This word names the early part of a life taken as one period, the way English speaks of someone's "young years" or "early days." It is almost always plural in form and almost always paired with a person: my youth, her youth, the youth of a people. It points less to a single age than to a whole formative stretch, the time when a person's habits, loyalties, and hardships first take shape.
Because it marks where a life begins, the Bible uses it as a starting line. A pattern is traced back to its source: faithful or sinful "from my youth," a skill or a hardship carried "since youth." Writers look back to it with regret over early sins, with longing for early devotion, or with gratitude for lifelong care.
The same idea is stretched over a whole nation. Israel's "youth" is its beginning as a people, the time in Egypt and the wilderness when the bond with God was new, remembered either as a season of first love or as the root of a long waywardness. From this early-life sense comes the tender phrase "the wife of your youth": the spouse married when both were young, named to call up the loyalty owed to a bond formed at the start.