Season 10, Ep 12
No process, offscreen, sorry.
Sam and Dean investigate mysterious disappearances and find out a witch is kidnapping them and turning them into teens. Along with Dean, a woman named Tina is AR'd
*Spoiler
Dean is restored but Tina isn't, though she's fine with it since she had a bad childhood and sees it as a second chance.
http://www.supernaturalwiki.com/inde...12_About_a_Boy
http://www.supernaturalwiki.com/index.php?title=Tina