For a variety of reasons, timelines on young adulthood cannot be exactly defined—producing different results according to the different mix of overlapping indices (legal, maturational, occupational, sexual, emotional and the like) employed, or on whether 'a "developmental perspective"... the "socialization perspective" is taken. 'Sub-phases in this timetable of psycho-social growth patterns... are not rigid, and both social change and individual variations must be taken into account'—not to mention regional and cultural differences. Arguably indeed, with people living longer, and also reaching puberty earlier, 'age norms for major life events have become highly elastic' by the twenty-first century.