HI Kim! YES YES YES!!! even a fetus can masturbate (and there are plenty of studies, images and whatnot attesting to this). Research does indicate, however, that the drive to masturbate is more prolific during puberty, and in early adulthood, than any other time; and that the average age of onset (keep in mind most of the research is based on white, heterosexual populations, or at least assumed hetero), is 13.
As a parent (and now grandparent) and someone who has provided childcare for others, I can personally attest that young children also touch themselves for pleasure, and that it seems to be a very subjective thing, that might have more to do with how they think than their age or development.
I also love your thoughts about consent. I would like to see this be an ongoing conversation at every level-for doctors on how to respect the bodily autonomy of their clients at all ages, for parents on respecting and teaching boundaries, for teachers on how to have classroom discussions.. Every age, every demographic!