Who would apter (2001) be most likely to call a "thresholder?"?

Someone between the age of 18 to 25 years.

Explanation:

In her book titled "Myth of Maturity," Terri Apter coined a term called 'thresholders.'

The term 'thresholder' is used to refer to those people who have legally achieved the title of adult but yet are stuck between adolescence and adulthood.

In her book, she points out the myths that parents believe about their children's maturity. She defined the people between the age group of 18-25 years are thresholders as they, though legally called adults, but they are plunged on the 'threshold' of adulthood.

So, Apter would most likely call someone a 'thresholder' who is between the age of 18-25 years.