I give you a picture that ESPN put up along with their article:
The year on the trophy seems to be 2002. That would make her about five years old in that photo.
So, from probably even
before that, she has been competing in tennis. For over 20 years.
I would imagine that, after all the time and effort spent from early childhood which is necessary these days to excel in professional sports, a person may just get
tired of it all.
If there is such a trend of athletes walking away early, I would say it is related to this relatively recent phenomenon of denying children normal childhood in favor of relentless training, practice, and pressure.