I recall one such study that claimed sexual activity brought on early puberty, in both girls and boys, and resulted in shorter stature as an adult.
The shorter stature correlates with earlier puberty regardless of the cause, because growth hormone levels decline as the reproductive hormone levels increase. That’s the kind of thing that’s easily studied.
As far as the cause of early puberty, there are a lot of elements to consider, and scientists can only investigate so much. Many hypotheses have some support, but far short of proof.