Neutron fluxes and currents in the vicinity of control rods inside reactors are calculated in age-diffusion theory, using a control rod in the form of an infinite plane slab and approximating its transparency by a simple analytical expression. The results are compared with a two-group calculation for the same geometry and indicate the degree of accuracy of the latter method. The comparison is made for epithermal neutron energies only, and shows that in this region, two-group theory is applicable for a calculation of the number of neutrons absorbed by the control rod with a fair accuracy.