We don't know what age grown up Buddy is. But we know at that point, it's possible for her to become a joy mutant.
Brad had been taking joy before he found Buddy, so it took him over a decade to turn into a mutant. Maybe the ending scene of joyful takes place 10+ years down the line too.
Then again, Yado was surprised she hadn't already mutated, so maybe it just works faster on girls.