You can look to a career path as two possible explanations. Traditionally, women have been viewed as having higher absenteeism and higher quit rates. More recent empirical evidence suggests that there is no gender difference in absenteeism and permanent quits, so the wage gap for a particular cohort may be stagnant or decline simply because those explanations are no longer used to account for women's lower wages for a particular cohort.
Another case in point would be women with children. As the children age, women are more able to devote themselves back to the labour force, so for a particular cohort, or birth cohort, the wage gap could be stagnant or decline because women's increased earnings will be reflected in their work effort.
Those are two reasons the wages are no longer diverging as workers age.