Given the importance of white dwarfs (WDs) in many fields of modern astrophysics, the precise knowledge of the actual degree of accuracy of the associated theoretical predictions is a primary task. In the first paper of a series dedicated to the modeling of WD structure and evolution we discussed the limits of the available theoretical studies of cooling sequences. In the present work we extend this analysis to isochrones and luminosity functions of WDs belonging to old stellar systems, like globular or old disk clusters. The discussion is focused on the most common DA, those with a CO core and an H-rich envelope. We discuss, in particular, the variation of the age derived from the observed WD sequence caused by different assumptions about the conductive opacity as well as that induced by changing the carbon abundance in the core. The former causes a global uncertainty of the order of 10% and the latter of about 5%. We discuss different choices of the initial-to-final mass relation, which induces an uncertainty of 8% on the GC age estimate.
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