He often talks about his adventures in his youth.

his 是形容词性物主代词，指代"他的"，表示所属关系。 youth 是名词，指"青年时期"或"年轻时代"，通常指人生中精力充沛、充满活力的阶段。 his youth 表示"他的青年时期/他的年轻时代"。