Frequency-dependent selection may favor the evolution of mixed reproductive strategies, resulting in polymorphic life-histories within a population. Here I explore the theoretical and ecological possibilities for evolution of mixed reproductive strategies in two minimal age-structured life-history models. The first model addresses evolution of delayed maturity (i.e. competition between annuals and biennials), and the second one deals with evolution of semelparity versus iteroparity. A necessary condition for evolution of polymorphism is that the description of environmental feedback in the model is two-dimensional. A two-dimensional description is necessary if different age-classes experience the influence from the environment differently and have a different influence on the environment. This might be caused by resource utilization or predation risk being different between age-classes.