The precursor of “tomboy” is hoyden, which Michele Ann Abate describes as follows:
First appearing in the late 16th Century, the term shares a similar etymology history: it also referred to rambunctions boys and men rather than girls and women. Ineed the Oxford English Dictionary provides the following definition for “hoyden”: “A rude, ignorant, or awkward fellow; a clown, a boor”. By the late 17th Century, however, this meaning shifted and the word began referring to like-minded members of the opposite sex: “A rude, or ill-bred girl (or woman): a boisterous noisy girl, a romp.” Unlike a tomboy, a hoden was more closely associated with breaching bourgeous mores than female gender roles.
She adds later:
Wen the concepts of “tomboy” make its debut during the mid-19th Century, it supplanted “hoyden.”
I think I’ve found the answer to my “what do you call a grown-up tomboy?” question: hoyden.