What's some really unpopular opinion you have?

Care for some actual science? You're making some extraordinary claims with very simplistic statements relating an interdisciplinary, highly complex field of research. There is not one single point in development when maturity is reached, there are different, simultaneous processes involving different aspects of development and maturity.

Searching for Signatures of Brain Maturity: What Are We Searching For?

(...) For the current discussion, the key point is that there is no single progression that encompasses functional maturation. Neural activity intensifies and reduces, varies quantitatively and qualitatively, in linear and nonlinear ways that are both linked to—and independent of—behavioral differences across development. Each of these patterns reflects developmental progress, but the wide range of ‘‘journeys’’ prohibits a simple definition of what emerging brain functional maturity looks like. (...)

Cognitive and affective development in adolescence

(...) As reviewed in the accompanying article by Paus [5] there is growing evidence that maturational brain processes are continuing well through adolescence. Even relatively simple structural measures, such as the ratio of whiteto-gray matter in the brain, demonstrate large-scale changes into the late teen-age years [6–8]. The impact of this continued maturation on emotional, intellectual and behavioral development has yet to be thoroughly studied, but there is considerable evidence that the second decade of life is a period of great activity with respect to changes in brain structure and function, especially in regions and systems associated with response inhibition, the calibration of risk and reward, and emotion regulation. Contrary to earlier beliefs about brain maturation in adolescence, this activity is not limited to the early adolescent period, nor is it invariably linked to processes of pubertal maturation (Figure 1). (...)

Behavioral and Neural Pathways Supporting the Development of Prosocial and Risk-Taking Behavior Across Adolescence

(...) Consistent with prior work showing that risk-taking behavior increases and peaks during adolescence (Gullone et al., 2000; Steinberg, 2007), we found that rebelliousness similarly increases from early adolescence to late adolescence before declining into adulthood. Research on the development of prosocial behavior however is mixed (for an overview, see Do et al., 2017). We observed a quadratic effect of age on a broad measure of prosocial behavior, peaking in mid-to-late adolescence, suggesting that, like rebelliousness, prosocial development follows a nonlinear age pattern that converges during late adolescence, although future studies should test if different age patterns are observed for different domains within prosocial behavior (such as helping and donating behavior). Our findings converge on the hypothesis that the development of rebellious and prosocial tendencies peak during late adolescence relative to earlier or later ages (Do et al., 2017), thus highlighting late adolescence as both a window of vulnerability and opportunity (...)