The study of software aging and rejuvenation is based on models that conjugate the complexity of architectural models with the problem of time dependence of parameters. Exploiting the metaphors of common performance-oriented modeling formalisms (such as Petri nets or queuing networks) with the support of proper solution techniques can help modelers in approaching the analysis of complex software-based systems. This paper shows how SIMTHESys (a multiformalism modeling framework) can be used to approach the modeling problem by implementing a new user-defined modeling formalisms and the related fluid-based solution engine.