Previous studies, using flow cytometry, have reported a lower platelet reactivity in neonates compared to adults. Only few studies were carried out in older children, and results were controversial in terms of age to reach adult platelet function. We studied a total of 125 healthy neonates, infants and older children, and 15 adults. alphaIIbbeta3 expression on resting and activated platelets was lower in all children, with an impaired capability of alphaIIbbeta3 activation (PAC1 and bound fibrinogen). This defect was observed until the age of fifteen with a gradual recovery with age. In neonates, we observed a defect of GPIalpha internalization, and demonstrated that this defect persisted in older children as well. In contrast with alphaIIbbeta3 integrin activation, we did not observe a gradual age-dependent recovery. These unexpected results point out the need for reference values in childhood.