The primary aim of the presented study was to identify the criteria (markers) of adulthood acknowledged by individuals in late adolescence and early adulthood, and to compare them with respect to quantity and quality (content). The second aim was to compare the criteria of adulthood indicated by both age groups with the ones provided by individuals representing the next developmental period, that is, middle adulthood. Furthermore, based on the obtained data a list of criteria of adulthood was compiled allowing for the examination of the indi-
vidual conception and sense of adulthood in adolescents and young adults. The study included 776 participants: 349 late adolescents, 335 young adults, and 92 individuals in their middle adulthood, who answered an open question concerning the conditions that it is necessary to fulfill to become an adult. The findings suggest that: 1) late adolescents focus on the objective criteria of adulthood more often than young adults; 2) on average, late adolescents and young adults define adulthood by indicating a lower number of criteria of adulthood than people in their middle adulthood; 3) compared with individuals in middle adulthood, late adolescents and young adults are less unanimous as to which attributes are important for reaching adulthood; and 4) people in their middle adulthood are found to indicate the quality aspect of fulfilling the roles and responsibilities characteristic of adulthood substantially more often than late adolescents and young adults do.