Blood stains can be crucial in reconstructing crime events. However, no reliable methods are currently available to establish the age of a blood stain on the crime scene. We show that determining the fractions of three hemoglobin derivatives in a blood stain at various ages enables relating these time varying fractions to the age of the blood stain. Application of light transport theory allows addressing the spectroscopic changes in ageing blood stains to changes in chemical composition, i.e. the transition of oxy-hemoglobin into met-hemoglobin and hemichrome. We have found in 20 blood stains that the chemical composition of the blood stain with age, called hemoglobin reaction kinetics, under controlled circumstances, shows a distinct time-dependent behavior, with a unique combination of the three hemoglobin derivatives at all moments in time. Finally, we employed the hemoglobin reaction kinetics inversely to assess the age of 20 other blood stains studied, again over a time period of 0-60 days. We estimated an age of e.g. 55 days correct within an uncertainty margin of 14 days. In conclusion, we propose that the results obtained under controlled conditions demand further evaluation of their possible value for age determination of blood stains on crime scenes.