Another thing to add in, adaptives come of age during a High, not during a 4T.
So I reckon whenever the next 1T begins, the cutoff line will be the last year any cohort was born contributed to the 4T climax. Could be 2000-2010. The media usually cuts Millennials off around 1993-2004. Generally speaking, the next generation starts to be born around 2-3 years before the next turning begins. This 4T started in 2008. Assuming this 4T lasts 21 years and we enter the High in 2029, that means those born in 2005 will be turning 24, thus coming of age. So 2005 seems to be a good point, for now.
So I reckon whenever the next 1T begins, the cutoff line will be the last year any cohort was born contributed to the 4T climax. Could be 2000-2010. The media usually cuts Millennials off around 1993-2004. Generally speaking, the next generation starts to be born around 2-3 years before the next turning begins. This 4T started in 2008. Assuming this 4T lasts 21 years and we enter the High in 2029, that means those born in 2005 will be turning 24, thus coming of age. So 2005 seems to be a good point, for now.