A closed form expression for the Laplace transform of lethargy-dependent neutron age τ ⊥(u) from zero lethargy to any lethargy u in a slab lattice of two materials, which are characterized by constant cross-sections, is obtained by solving Fermi age equation with a plane neutron source at the midplane of one of the slabs of an infinite lattice. Due to complexity of the Laplace transform obtained for τ⊥(u), numerical inversion is carried out to obtain (a) neutron age from 2 MeV to indium resonance energy 1.45 eV in a number of Al-H2O lattices ranging from pure aluminum to pure water and (b) neutron age as a function of lethargy in 5-5 cm AI-H2O lattice. The results obtained are in satisfactory agreement with the existing literature in those few cases in which experimental or Monte Carlo values are available. At the same Al-H2O volume ratio, neutron age is found to increase or decrease with increasing plate thickness depending on the neutron source location in aluminum or water respectively. Furthermore, everything remaining the same neutron age is smaller with the source in water than in aluminum.