Glycated Hb was measured in reticulocytes (RNA-positive cells) and was correlated to a decrease in RNA fluorescence intensity in a Log-linear dependent manner, with no statistically significant differences in slope or in the ordinate of the origin between individuals. The loss of RNA fluorescence intensity in reticulocytes observed by flow cytometry is directly related to the time they spend in circulation and consequently to their lifespan. By setting the 10 time that reticulocytes spend to mature in the blood circulation and because the relation between the decrease in log RNA fluorescence intensity and the increase in log HbA1c fluorescence intensity is linear, the inventors are able to calculate the increase in HbA1c fluorescence intensity depending on the time and thus to determine the mean half-life of a RBC population. This new tool is noninvasive and requires only one single time point measurement. 15 The present invention relates to a method for in vitro determination of the age of red blood cells of a set of red blood cells by using intracellular glycated hemoglobin measurement by flow cytometry and by using a standard curve generated from glycated hemoglobin measurement in reticulocyes.