Their Belief in Allah and their Retreat from their People

From here Allah begins to explain the story in detail. He states that they were boys or young men, and that they were more accepting of the truth and more guided than the elders who had become stubbornly set in their ways and clung to the religion of falsehood. For the same reason, most of those who responded to Allah and His Messenger were young people. As for the elders of Quraysh, most of them kept to their religion and only a few of them became Muslims. So Allah tells us that the people of the cave were young men. Mujahid said, "I was informed that some of them wore some kind of earrings, then Allah guided them and inspired them to fear Him, so they recognized His Oneness, and bore witness that there is no god besides Him."

وَزِدْنَـهُمْ هُدًى

Commentary

The word: فِتْيَةٌ(fityah) in: إِنَّهُمْ فِتْيَةٌ (They were young men - 13) is the plural of: فَتٰی (fata) which means someone young. According to Tafsir scholars, this word indicates that the time ripe for correction of deeds and morals, and the inculcation of guidance and righteousness, is invariably the time when one is young. When old, formerly acquired deeds and morals become so deeply rooted that - no matter how evident becomes the truth against these - it is very difficult to break loose from their shackles. Those among the noble Companions who responded to and believed in the call of the Holy Prophet ﷺ were, after all, mostly young people. (Ibn Kathir, Abu Hayyan)