Answer
What happened was Jay's epiphyseal, growth, plate-bone junction has separated from one of the bones. This would not happen to Trixie, Jay's 23-year-old sister because her epiphyseal plate or growth place has already closed up.
Work Step by Step
The growth plates close up after the final growth spurt which can happen between ages 16 and 21 so Trixie has surpassed the closing of her growth plates by 2 years.
