The photons of the emitted light must have higher energies than the excitation photons in order to avoid measurement of ordinary photoluminescence. A sample in which the mineral grains have all been exposed to sufficient daylight (seconds for quartz; hundreds of seconds for potassium feldspar) can be said to be of zero age; when excited it will not emit any such photons.
For emitted light photons to avoid ordinary photoluminescence, they must have higher energy than the excitation photons. A sample where all mineral grains have been exposed to sufficient daylight (seconds for quartz; hundreds of seconds for potassium feldspar) can be considered ageless; when excited, it will not emit these photons.