there is no mental disorder where a person identifies as a 6 year old rather than their biological age. there is a mental disorder where a person identifies female or male in the opposite gendered body (gender dysphoria)

there is also a mental disorder where a person has two or more alternate personalties that take control with their own sense of agency and such (the actual diagnostics are in my link at the bottom of my post)

but no mental disorder where a person identifies as a 6 year old in an adult body, for example like a trans gendered person considers their self to be male or female in the opposite gendered body

that said

....Here in my location.....there is a completely normal phrase for people who identify as a child in an adult body....Peter Pan syndrome and Female Peter Pan Syndrome. its not a mental disorder. its a situation where an adult identifies\feels\prefers\ wants to remain a child despite having an adult body.

to find out what this is called in your own location you will need to contact your own treatment providers.