Adolescence is a transitional stage of physical and mental human development that occurs between childhood and adulthood. This transition involves biological (i.e. pubertal), social, and psychological changes, though the biological or physiological ones are the easiest to measure objectively. Historically, puberty has been heavily associated with teenagers and the onset of adolescent development. In recent years, however, the start of puberty has seen an increase in preadolescence and extension beyond the teenage years, making adolescence less simple to discern.