Why we can't determine reliably the age of a subject on the basis of his maturation degree.

•The concepts of maturation and biological age are critically reviewed.•Biological maturation is used to estimate the age of adolescents of unknown age.•Biological age is a measure of degree of maturation, not of chronological age.•The reliability of the estimate crucially depends on biological variability.•Due to this, the uncertainty interval of the estimate cannot be lower than ±2 years.