When she's still in her first year of the experiment, which we both know went really poorly
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Don't think there's any reason for her to go back to grown-up self if she's continuing the experiment. (And I don't think it was the end of the experiment yet at this point in time so all the more so)
Also we know grown-up Hishiro has a different (and prettier) hairstyle at the end. xD