Biological development of children and youths is undergoing microevolutionary changes in the 21st century as a consequence of changes in health care, nutrition and family environment. The most striking of those changes is the acceleration of growth and maturation producing adult bodies in early teenagers that, in modern systems of prolonged education and upbringing, are socially immature. This mismatch of adult bodies, including sexual behaviors, with a continued education and social dependence may produce health risks and psychological and social instabilities in formation of personal relationships and families. It may also generate difficulties in socialisation and workplace relations. Increased use of psychotropic substances, criminal behaviours and addictions are likely to develop when physically mature individuals have no opportunity to participate in normal adult life.