The question is almost impossible to quantify. Newborns can survive longer periods of decreased oxygen because they have a large proportion of fetal hemoglobin in the circulation. The oxygen saturation curve is shifted to the left. Small children over six months of age who asphyxiate are not so lucky. There are just so many variables that it is impossible to predict outcomes in most cases of hypoxia, especially in infants and small children. I've seen infants with five minute Apgar scores of zero do perfectly well afterwards, while those with higher scores sometimes fare much more poorly. OK, sure, if pupils are fixed and dilated and the Glasgow coma score is 3 or 4, the prognosis is grim. Still, timing the period of anoxia with certainty is nearly impossible, and correlating it with outcome is still an educated estimation.