‘Abdullāh ibn ‘Amr ibn al-‘ās (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "He does not belong to us who does not show mercy to our young and honor our old."
A Muslim is not compliant with the Sunnah of the Prophet (may Allah's peace and blessings be upon him) if he is not merciful to the younger members of the community by treating them with compassion and jolliness. The same is true if he does not treat the older members of the community with due respect and veneration. The expression "he does not belong to us" is intended as a warning and threat, not that such conduct actually takes the person out of the fold of Islam.