The organ most in need of oxygen, and therefore the easiest and most extensively damaged by lack of it, is the brain.
After just 4 to 5 minutes without sufficient oxygen the human brain is seriously damaged. Those who survive past that point offer suffer life long handicaps, both physical and mental.
Miracles do happen however. When it comes to drowning children young age, the so-called diving reflex and the temperature of water is often given as a reason of their possible unharmed survival.
The idea is that a young child’s diving reflex (shutting off the intake of air so that no water comes in) coupled with their small size and the coldness of the water slows down their metabolism sufficiently enough that the brain needs less oxygen. Think of it as time slowing down, stretching out, so that the about 5 minutes that we normally can’t go without oxygen extends to 30 minutes and maybe longer.
An example of a child who survives drowning is Oluchi Nwaubani (2) who was submerged for 30 minutes.
Still, children survive without the condition of cold water too. 3 year old Keegan Kinsley choked for at least 9 minutes and lived.
And premature babies can go without sufficient oxygen and make it. Angelina Mussini was born at 27 weeks and at one point didn’t breath on her own. Doctors warned for brain damage from lack of oxygen. But she made it through.
Likewise adults have been known to suffer from extensive oxygen deprivation and make it. Often this is with medical care and attention, of course. Leslie Williams went without oxygen for 30 minutes, was “frozen” by a doctor and came back to life.
Julie Bromage was dead for 15 minutes, no oxygen to the brain, and pulled through.
Additional Reading & Miracles
You can learn more about oxygen deprivation at Wikipedia.
To better make sense of some of the amazing oxygen deprivation survival stories, see also our extended entry on miracles.