Previous calculations and measurements of the age of fission neutrons in water are tabulated with special emphasis on those aspects of the experiments which may require corrections or lead to errors. A plane-source geometry is described and data taken in this geometry are interpreted in terms of these corrections. The measured second moments of the distributions from 18, 24, and 30 in. sources are corrected for foil and source plate mass effects. A geometric correction is then applied to obtain the age of fission neutrons in water as 27.86 ± 0.10 cm2.