This work continues our efforts to calibrate model surface brightness fluctuation luminosities for the study of unresolved stellar populations, through a comparison with the data of Magellanic Cloud star clusters. We present here the relation between absolute K_s-band fluctuation magnitude and (V-I) integrated colour, using data from the Two-Micron All-Sky Survey (2MASS) and the Deep Near-Infrared Southern Sky Survey (DENIS), and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by Liu et al. We find that intermediate-age to old star clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the versus (V-I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V-I_C)₀ < 1.25, ours holds in the interval . This implies, according to Bruzual-Charlot and Mouhcine-Lançon models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, a slight offset between the galaxies and the star clusters [the latter are ~0.7 mag fainter than the former at a given value of (V-I)], caused by the difference in metallicity of roughly a factor of 2, confirms that the versus (V-I) plane may contribute to break the age-metallicity degeneracy in intermediate-age and old stellar populations. The confrontation between models and galaxy data also suggests that galaxies with K_s fluctuation magnitudes that are brighter than predicted, given their (V-I) colour, might be explained in part by longer lifetimes of thermally pulsing asymptotic giant branch stars. A preliminary comparison between the H 2MASS data of the Magellanic star clusters and the sample of 47 early-type galaxies and spiral bulges observed by Jensen et al. through the F160WHubble Space Telescope filter leads to the same basic conclusions: galaxies and star clusters lie along correlations with the same slope, and there is a slight offset between the star cluster sample and the galaxies, caused by their different metallicities. Magellanic star clusters are single populations, while galaxies are composite stellar systems; moreover, the objects analysed live in different environments. Therefore, our findings mean that the relationship between fluctuation magnitudes in the near-infrared, and (V-I) might be a fairly robust tool for the study of stellar population ages and metallicities, could provide additional constraints on star formation histories, and aid in the calibration of near-infrared surface brightness fluctuations for cosmological distance measurements.