There’s no objective or clinical sense in which an adult—Donald Trump included—“behaves like a six-year-old child.” That phrase is usually a metaphor, not a literal assessment.

What people often mean when they use it is this:

• Communication style: Trump frequently uses simple language, repetition, nicknames, and personal attacks, which some critics interpret as immature or impulsive.
• Emotional expression: He tends to respond publicly and forcefully to criticism, showing anger, defensiveness, or boastfulness rather than restraint.
• Decision-making approach: Supporters see this as confidence and authenticity; critics see it as impulsivity or a lack of deliberation.