Acting much younger than one's age is the defense mechanism of:

1. Each defense mechanism has a distinctive outward sign. 2. In regression, an individual acts much younger than their age. 3. Projection instead relocates a feeling onto another person. 4. Repression keeps material out of awareness altogether. _Source: OpenStax Psychology 2e (CC BY 4.0), Ch 11 'Personality', sections 11.1-11.9_