Which would be the consequences of untreated precocious puberty once they are a grown adult?

Complicated: You don't specify in girls or in boys, many possible answers depending on the cause of the precocious puberty. Short stature/height would be nearly universal, i'd need more specifics to say more.

It depends: Is this precocious puberty in a boy or girl? Age of diagnosis? Untreated precocious puberty in a 4 yo girl is much more likely to be a problem than for a 7 year-old. The younger the child, the more likely a reason to get a brain MRI looking for something pushing on the pituitary, particularly in a boy. Final height might be compromised; most very young girls not ready for periods, etc.

Small stature: Untreated precocious puberty is caused by post-pubertal hormone levels, Estrogens in girls & testosterone in boys. Results in early development of adult bodies, including genitalia, hair & breasts. Allso experience early fusion of bony growth plates. Thus bones stop growing & subjects although taller & bigger than peers at first end up much shorter as adults.Also pituitary gland adenomas(growths).