From about what age should I reduce a parent's help with things like bathing and dressing?

There's no uniform "from what age"; reduce it gradually to match the child's skills and growing sense of privacy. Break the process into steps, show the order of washing or dressing with pictures, and leave the parts they can do. Toward adolescence, widen the range they do alone alongside learning about body growth and private zones. Secure safety (supervision in the bath) while shifting in stages: supervise, then prompt only, then do it themselves. Without rushing, build up the child's "I did it."