From both:
Age Unconfirmed[1]
[1] In The Birth of Mewtwo, Jessie’s mother Miyamoto wrote a report 20 years before Mewtwo’s creation that says she gave away her daughter to be a foster child as a little girl. Jessie is affected by the same lethargy as other adults in Filling the Light with Darkness!
so Jessie would presumably be at least 20, probably early 20s.
2 points
