A simple and fast method is proposed for designing and optimizing broadband and narrowband terahertz (THz) graphene absorbers in the desired frequency bands or any number of desired frequencies. Also, the maximum bandwidth for multilayered structures is proposed by this method. This method uses a genetic algorithm, transmission line theory, and impedance matching technique. The proposed THz absorbers are perfect frequency-tunable structures with absorption values more than 90%, which consist of multilayered graphene separated by a dielectric material. Continuous graphene is utilized in the structure of these absorbers but metamaterial graphene can also be used. The optimization parameters are the thickness of dielectrics, refractive indices of dielectrics and the chemical potential of the graphene layers. These graphene-based absorbers have high potential for different THz applications such as filters, detectors, switches, modulators, etc.