Women, the elderly, children, sick الحجاج, and those who are weak or have difficulty with crowds are permitted to leave مزدلفة after midnight (or after the moon sets) عند the consensus of scholars from all four schools. This is a confirmed السنة concession granted by النبي (صلى الله عليه وسلم) himself. He sent the weak members of his family, including his wife Sawdah and others, ahead from مزدلفة to منى during the latter part of the night to avoid the rush of the main body of الحجاج departing at dawn.
الشيخ ابن باز explained that the concession is available to all who have a genuine reason for early departure, including women who want to avoid the extreme crowding, elderly الحجاج, those with small children, and the sick. These الحجاج may proceed directly to منى and stone Jamrat al-Aqabah before dawn, which is جائز for them based on this concession. Strong, able-bodied men should remain at مزدلفة until after Fajr الصلاة, following النبي's own practice.
الشيخ ابن عثيمين specified that the earliest جائز time for departure is the latter half of the night (after midnight), based on the narration that النبي sent them ahead 'at the end of the night' and the moon setting. He cautioned against departing too early — shortly after Maghrib and Isha الصلوات — as this would mean the الحاج barely stayed at مزدلفة at all. Those who take this concession should still pray Maghrib and Isha at مزدلفة before departing.