There is a verse in the Qur’ān which says that before the age of forty we are not responsible for our deeds. Does that mean we will all go to heaven if we die before forty?
No I am afraid this is not the correct conclusion. If the style, stress and context of the verse (46:15) are given due consideration, it means that till forty years, if a person is living in a bad society or in the worst of circumstances, the Almighty will treat him leniently as far as his religious responsibilities are concerned. However, after this age no allowance will be given to him if he still remains indifferent.
The real age in which one’s responsibilities start is the age of one develops the cognisance of good and evil. This is different for different people and varies in the circumstances they live in. Forty years, it seems, is the last limit of attaining this cognisance fully. This also does not mean that it is forty years for every person. It means that, in human beings in general, this is the latest age by which a person should mend his ways.