Adultish is the correct term for behaviour frequently and incorrectly described as 'childish'.
Hitler is often called 'childish', as if his misdeeds were those of children in general, rather than of one highly specific adult. Hitler wasn't childish. He invaded Russia! That takes a great deal of organisation, even if the operation ultimately backfired; far more tactical expertise than any mere child can boast. History records the actions of very few children. Never has there been an authenticated case of a child invading Russia or any other nation. For one thing, they would have to be home in time for tea!
It might be pointed out at this juncture that 'childish' is a reference to Hitler's petulance, not to the logistics of his campaigns. Nonetheless, real petulance and greed are refinements of the adult. When we casually employ the term 'childish' to dismiss grown up people, we are committing an injustice, for children actually have fairly clean motivations and few requirements. They want attention; unconditional love, entertainment, automatic forgiveness, presents. That's all. They almost never demand justice, sovereignty, empowerment, enfranchisement, hegemony or revolution. Their schemes are modest and small scale. It may be pointed out that those schemes are of the same type: that a child's desire to take a coveted object, say a toy, away from another child is the same as a dictator's desire to deprive another ruler of a country, say Russia. It can be insisted that the difference is quantitative, not qualitative.
But in such cases quantity is inextricably linked to quality and adjusts the quality as a consequence. A toy is an insignificant thing really; Russia is not. To be whacked by a stick once is qualitatively no different from being whacked a hundred times: so runs common wisdom. The difference is only quantitative. Anybody who accepts this should subject themselves voluntarily to a hundred blows and then judge the lack of difference in quality.
When we utter the insult 'childish' we are in fact objecting to actions or utterances that are properly and profoundly adult in nature. The correct word to describe examples of selfishness, power hunger, bursts of anger, the bearing of very long grudges, the attempted manipulation of psychology, and even the glib vocalisation of inappropriate responses, is Adultish.
Adult behaviour rarely has much to do with maturity. The average adult is not mature. The average adult is not childish either, but far worse than that. The average adult is Adultish.
This depiction of children as innocents who are satisfied with the basic emotional necessities of life may appear artificially naïve. Children twist the arms of playmates; they pull legs off spiders; they are bullies. But who excels at such behaviour? Who acts that way on a vastly bigger scale? Adults. Rather than label adults who indulge in such cruelty 'childish', surely it's more accurate to say that aggressive and cruel children are exhibiting precocious Adultish traits!
A fully-grown man or woman who rants and raves, who is selfish, glib or petulant, grasping, egotistical, isn't acting like a child; on the contrary, a child in the middle of a tantrum is practising for adult life. The upshot of all this is hardly profound but is often conveniently forgotten: the normal adult human being is a monster.
It is nicely symmetrical to state that there is only one thing that is 'childish' when an adult does it, namely accusing other adults of being 'childish' in an attempt to belittle them. That truly is childish... But in fact adults do this all the time and children never do it, so it can't be childish; it must be Adultish instead.