It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The best of your young people are those who resemble your middle-aged people (in seriousness and dignity), and the worst of your middle-aged people are those who resemble your young people (in carelessness)."
Hadith Referenceالمعجم الأوسط / باب الميم / 5904
Hadith Takhrij«أخرجه البزار فى «مسنده» برقم: 6944»