  In previous papers of the series we used a combination of optical and
near-infrared colours to derive constraints on the relative age structure in
globular cluster systems. Here, we present the details, strength and
limitations of our method based on Monte-Carlo simulations of colour-colour
diagrams and cumulative age distributions. The simulations are based on general
informations about the globular cluster systems (e.g. colour-ranges, the number
ratios between sub-populations) and the different single stellar population
models (SSP's) which are used to derive relative ages. For both the modeled
systems and the observed globular cluster systems we derive the cumulative age
distribution and introduce two parameters to define it, the so-called 50 % age
and the result of the reduced $\chi$$^2$test of the comparison between models
and observations. The method was tested successfully on several systems and
allowed to reveal significant intermediate age populations in two of them.
