Medial longitudinal Arch age stablization.

It seems to depend on what source you read as to when the MLA (medial longitudinal arch) stabilizes, but here is a number, between 7-9 years old. 

Conclusions: The MLA of children remained stable from 7 to 9 years old. Gender and the type of footwear worn during childhood may influence MLA development.

Reference:

Medial Longitudinal Arch Development of Children Aged 7 to 9 Years: A Longitudinal Investigation.Jasper W.K. Tong, Pui W. Kong Journal of American Physical Therapy Assoc.   DOI: 10.2522/ptj.20150192 Published February 18, 2016

http://ptjournal.apta.org/content/early/2016/02/17/ptj.20150192

Development of the arch: Functional implications | Lower Extremity Review Magazine

A nice, referenced piece from one of our fav’s, Dr Michaud.

“Although early research suggested a limited connection between arch height and lower extremity function, more recent research confirms that arch height does indeed affect function. Information obtained from measurements that accurately identify the height of the medial longitudinal arch may lead to more effective treatment protocols. By identifying specific injuries associated with low and high arches, it may also be possible to prevent these injuries.”

Development of the arch: Functional implications | Lower Extremity Review Magazine