I agree with alpha on this one. You can fly to altitude without Blacking out instantly. Unfortunately the 1st signs of hypoxia are hard to notice. Which is why in the US they require all pilots to use it well before truly necessary. Many thing will influence when hypoxia will start to take effect on you. If you live in the mountains, 7000 msl, you will more then likely be able to climb higher than a pilot of the same shape how lives at sea level. Drinking alcohol will drastically increase your chances of hypoxia. Having a beer in your system from the day before will still effect you. And lastly being in good shape will help you.
The required altitudes the FAA for oxygen is a catch all. It's better to have everyone breathing early that too late.
Read the Book. Fighting the flying circus. In WW1 there were pilots flying over 20000 feet without oxygen! I can only imagine the feeling of a open cockpit bi plane at that altitude.