Composite concrete beams made of prefabricated prestressed element and cast in place reinforced concrete slab become very popular in nowadays bridge engineering. The two concrete composite parts are cast at different times, under different conditions. As a consequence, they have different concrete strengths and moduli of elasticity, as well as different rheological properties. The latter is responsible for stress redistribution within the composite section. The objective of the paper is to evaluate the influence of different concrete age between the girder and the in-situ slab on the behavior of composite concrete beams. Numerical study was performed on a real example of composite beams that are part of a multi-span continuous highway bridge. Four age differences between the precast girder and the in-situ slab were considered: 30, 90, 365 and 730 days. The numerical results indicate that different creep and shrinkage properties arising from the age difference between the concrete parts can significantly affect the stress redistribution, as well as the final deflections of the composite concrete beams.