Age regression is a psychological phenomenon in which an individual reverts, $8 mentally speaking, to a earlier age. This can be a conscious choice during a course of therapy or an involuntary reaction to stress. For some people taking comfort in nostalgia returns them to that time before certain stunning tragic events.
Other people may acquire behaviours and emotional states which are those of younger phase in development. It is, however, important to note that this is distinct from chronological aging and does not imply any kind of physical reversion.