The Youngest moon that has been observed after the New Moon is about 12.5 hours old, but it is very difficult to see because it is so close to the Sun.  You can find the young Moon soon after the Sun's limb sets below the horizon, or the old Moon just before the sun rises.  An exact ephemeris must be calculated at each location on the Earth to see which sites give the youngest Moon viewing chance every month due to its rapid location and time change.