The intrinsic uncertainty (due to statistical effects) affecting the integrated colours and mass-to-light ratios as a function of the cluster's integrated visual magnitude (M_V^tot^_) is studied. We investigate the case of metal-poor, single-burst stellar populations with age from a few million years to a likely upper value for the Galactic globular cluster ages. Monte Carlo techniques are used for randomly generating stars distributed according to the cluster's mass function. Integrated colours and mass-to-light ratios in different photometric bands are presented as a function of age for different assumptions on the cluster total V magnitude and they are checked for good agreement with the observational values of low-metallicity Galactic clusters.