Can you explain if it's possible for a girl's height to increase after the age of 18?

Unlikely to happen: The growth centers of both genders are essentially closed by the age of 18 making any more linear growth unlikely. An x-ray of the hand can determine if any growth centers remain open, but at 18 this is unlikely.If the x-ray showed bone maturity to be more consistent with the age or 13-14, that much growth might be possible but a visit with an endocrinologist would be needed to understand why.