Time-delay systems are ubiquitous in nature and engineering. Delays account for transmission, diffusion and migration phenomena as well as maturation and aging in physical, biological, ecological, epidemiological and population systems. Lotka-Volterra models describing prey-predator models or Pearl-Verhulst equations for population dynamics are instances of deterministic models aiming to describe population evolution. These models do not account for aging/maturation of the population and this is the reason why refinement of the models have been proposed.