Estimating the forensic age of living individuals is ever more important in forensic practice, due to the ongoing increase of migratory flows, amongst other causes. Using the Greulich and Pyle method on a sample of 1150 individuals of the Spanish population (n = 560, 0-18 years for girls, and n = 590, 0-19 years for boys), the mean difference between the bone and chronological ages was obtained: 0.01 years (- 0.81, + 0.92) for girls and 0.33 years (- 1.15, + 0.34) for boys. For a same class of age and sex, the inherent variability was also evaluated: [Formula: see text] (0.41-1.25) for girls and [Formula: see text]years (0.36-1.76) for boys. To minimise systematic errors with regard to the reference population, adjustment factors are proposed for each age and sex. A sequential classification criterion based on decision trees is postulated to improve reliability in the prediction of maturity. Implementation of the decision criterion in three categories enables the doubtful individuals to be separated into the category of "undetermined" and to satisfactorily classify in the categories of "mature" and "under age": 0.96 (0.86-0.99) specificity; 1.00 (0.92-1.00) specificity; and 1.00 (0.92-1.00) predictive value.