Simple myopia usually starts in childhood, it increases more rapidly in adolescence and usually begins to stabilise itself at the end of body development (18-22 years).
High myopia usually increases more rapidly and stablises later in life. It's difficult to determine a set age as it will depend on many factors, the most important probably being the number of dioptres, i.e., the higher the myopia, the longer it will take to stabilise. The majority of myopias do not usually increase consistenly past the age of 25, but a random increase beyond that age cannot be ruled out.