Natural, current, recognisable
Young performers change quickly. Their photographs need to keep up.
With children and teenagers, a headshot can become outdated much faster than it might for an adult. Hair changes, faces mature, teeth change, height and proportions alter and the overall impression of the young performer can move on surprisingly quickly.
In practice, I often see the same young performer again as they grow. There is no arbitrary timetable that suits every child — the important question is whether the photograph still looks like them now.
That matters particularly when the image is being used on a casting or agency profile, because the photograph should help create recognition rather than surprise.