I have answered this question before. The answer is that the youngest first crescent Moon after New Moon is about 18 hours old and very hard to see because it is so close to the Sun that you have to look for it just as soon as the Sun's limb sets below the horizon. Many amateur astronomers have competitions internationally to see who can see the youngest Moon. Its location and time changes very rapidly, and an exact ephemeris has to be calculated at each location on the Earth to see which sites yield the youngest Moon viewing opportunity every month.