I could try to use the coin to change reality

Tom eyed his young and now very sexy wife. Whether due to the last wish or her own desires, she was clearly excited by the changes. But he could still read Sarah very well and see that she was disappointed by the small "scope" of the de-aging.

"Do you want me to use the coin to try to change reality?" he offered. "To actually change everything so that you're actually 18?"

Her eyes widened and she shivered with anticipation. "Yes!" she gasped, 'Do it!"

Grasping the coin he made his wish. "I wish that everything changed to reflect Sarah's new age of 18 years old"