Personality regression is a mental state in which one suddenly adopts an identical or similar personality, thought structure, mannerisms and behaviours to that of their past self from a younger age.    During this state, the person will often believe that they are literally a child again and begin outwardly exhibiting behaviours which are consistent to this belief. These behaviours can include talking in a childlike manner, engaging in childish activities, and temporarily requiring another person to act as a caregiver or guardian. There are also anecdotal reports of people speaking in languages which they have not used for many years under the influence of this effect. 
Personality regression is often accompanied by other coinciding effects such as anxiety, memory suppression, and ego death. It is a relatively rare effect that is most commonly induced under the influence of heavy dosages of hallucinogenic compounds, such as psychedelic compounds, such as LSD, psilocybin, and mescaline. However, it can also occur for people during times of stress,  as a response to childhood trauma,  as a symptom of borderline personality disorder, or as a regularly reoccurring facet of certain peoples lives that is not necessarily associated with any psychological problems.
- Lokko, H. N., & Stern, T. A. (2015). Regression: diagnosis, evaluation, and management. The primary care companion for CNS disorders, 17(3). | https://dx.doi.org/10.4088%2FPCC.14f01761
- Regression definition | https://psychcentral.com/encyclopedia/regression/
- Regression definition (AlleyDog) | https://www.alleydog.com/glossary/definition.php?term=Regression
- Fromm, E. (1970). Age regression with unexpected reappearance of a repressed c3ildhood language. International Journal of Clinical and Experimental Hypnosis, 18(2), 79-88. | https://doi.org/10.1080/00207147008415906