As a general rule, for children under the age of 16 years, every year or every whole shoe size change as in foot growing from a size four to a size five.  After age 16, general rule is about once every two years.   From age 20, it changes to about once every three to five years which is of course only necessary if there are still issues of instability or pain.  With that being said, if the child's bone structure and or mechanics and instability are positively affected by the orthotics to the degree of becoming stable, the orthotics are only needed if the child/young adult is more comfortable wearing them than not.