Some form of Applied Phlebotinum results in one or more main characters regressing to an earlier age. It may be the result of a freak accident, a plot on the part of the bad guys to incapacitate our heroes, or done on purpose so the characters can relive their childhood (at least temporarily).
For some reason this plot is often used in Fan Fic, where it's called a De-Aging story. Often combined [...] [tvtropes.org]