"Allah is the One who created you from weakness"—meaning: the sperm-drop of the man—"then He appointed after weakness, strength"—meaning: youth.
﴿اللَّهُ الَّذِي خَلَقَكُمْ مِنْ ضَعْفٍ﴾ يَعْنِي: نُطْفَةَ الرَّجُلِ ﴿ثُمَّ جَعَلَ من بعد ضعف قُوَّة﴾ يَعْنِي: الشبيبة.