The Tanner scale measures sexual development. Nothing else. It has nothing to do with general maturity, it just measures if the external sexual characteristics have come in.

This two-minute video can explain it to your muddled mind:

https://youtu.be/tCaPPyQda04

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 (...)

We've not changed the way we see adolescent development for far longer than mental health and suicide have been rising, meaning your ideas of what's contributing aren't even remotely the cause. You wanna know why teenagers are killing themselves in record numbers? Here's a few quick reasons...

1. Social media. Influencers have created an entire fake reality that is so impossible for the average person to achieve that they feel disconnected and worthless. The fact that these people make boatloads of money while producing and providing zero value to the economy is also disheartening.

2. Climate change. It's clear that it's happening, and clear that governments and their corporate overlords are willing to destroy the fucking planet while we can't do anything about it.

3. Cost of housing. Want to retire? Ever? Hahahahahahahaha hahahahahahahaha. Hope your family is wealthy or has a place to pass down to you.

4. Cost of education. In the last 50 years the cost of a bachelor's degree has risen 500% above inflation. Somehow this has also happened during a decreased value of education as well (so literally breaking the economics of supply and demand) because every fucking entry level paperclip counting job needs you to have a 60k degree to "prove you have the discipline to get things done."

But yeah, I'm sure it's the fact that 13 year olds aren't considered legal adults (something that's never been a thing in our society) that's somehow causing the increased suicide in the last two decades. Cause that makes any fucking sense at all.

I didn't want to accuse some stranger of that, but he makes it really hard not to think so.