The 85% number seems to have been quoted for at least a decade or so, cited in some cases I can find to a 2013 book [1]. Here's a summary via a 2019 open-access

The 85% number seems to have been quoted for at least a decade or so, cited in some cases I can find to a 2013 book [1]. Here's a summary via a 2019 open-access book chapter about leaving the Amish [2]: > Contrary to the persistent myth that they are dying out, the Amish population in North America continues to grow at a rapid pace. Numbering over 330,000 in thirty-one states and four Canadian provinces, the Amish population doubles every twenty years (Young Center 2018; Donnermeyer et al. 2013). While over 60 percent of the Amish still reside in Pennsylvania, Ohio, and Indiana, home to the four largest Amish settlements, population pressures are increasingly resulting in out-migration to areas where land prices are more affordable. Because the Amish do not actively seek converts, this population surge is a result of two major forces—large family sizes and a high retention rate. Though family size differs by affiliation, Amish families still average approximately five children. At the same time, the retention rate is at an all-time high. Fully 85 percent of Amish youth get down on their knees in front of their congregation and pledge to uphold the Ordnung, or unwritten code of conduct, of their local church district (Kraybill, Johnson-Weiner, and Nolt 2013). [1] Kraybill, D.B., Johnson-Weiner, K.M., and Nolt, S.M. (2013). The Amish. The Johns Hopkins University Press. [2] McConnell, D.L. (2019). Leaving the Amish. In: Handbook of Leaving Religion, ed. D. Enstedt, G. Larsson, T. Mantsinen, pp. 154-163. https://doi.org/10.1163/9789004331471_013 https://doi.org/10.1163/9789004331471_013