Here's a list of women who have won a Grand Slam tournament in the modern era when they were her age or younger:
Evonne Goolagong
Chris Evert
Tracy Austin
Han Mandlikova
Steffi Graf
Arantxa Sanchez-Vicario
Monica Seles
Martina Hingus
Iva Majoli
Serena Williams
Maria Sharapova
Svetlana Kuznetsova
Jelena Ostapenko
I'm not arguing that this necessarily proves anything, but Ostapenko joins an extremely impressive cohort of 13 women, everyone of whom, except for Iva Majoli, is in the Tennis Hall of Fame or will be locks to do so the moment they retire (Sharapova may have to be patient, but she is a lock based on her record on the court).
That being said, I hope winning so early doesn't go to Jelena's head. I don't think she is close to realizing her full potential, and her second serve remains a severe liability. Also, her gung-ho, do-or-die style, which is very exciting to watch, needs to be modified a little bit if she is to win consistently. So there is a lot of work to do. I would assume she might struggle for awhile. But there is also the chance that she could end of being something special.