We don't have in Islamic texts a specific age in which period of youth ends. Period of youth depends on the state of mind more than state of body. Ameerul Mo'mineen Ali (AS) remained young (Fata) for all his life. People of the Cave are called in Quran as young (Fityah) but their age was long. Ammar ibn Yaasir was an active fighter in the army of Ameerul Mo'mineen in Siffeen battle despite the fact that his age was over ninety years. Youth remains as much as its spirit is their and it is not limited to certain age.
'Wassalam.