Actually the word is KIDULT. A kidult is a grown up person who enjoys being part of a youth culture and doing things that are usually thought as more suitable for children like playing with toys,watching cartoons,etc.

There are no such words. but yes, albeit hesitatingly but I should confess, I have heard people using this word KIDULT. But this is in no way a standard word and if used only proves the lack of vocabulary of the speaker. I will never use kidult and I shall also never let my students use it but well, that is of course my personal choice. Hope this helps. :)

Not a word at all in English.

No, this is not a word in America. I would not understand it even if someone spoke the word. When foreigners learn how we speakers of English can make contractions and form a new word, they think they can do the same, but it doesn't work when you are not a native speaker. The term we understand is "childish adult". The word "childish" describes the behavior of the adult. We might say "he is just a kid at heart" or "some people never grow up"! or "he acts so immature". We might say "he enjoys childish games" or he is still fascinated with children's toys. But if a person is emotionally childish, "throws a tantrum" or cannot control their emotions like an adult, we call it "childish" behavior.

This would be a new word - but it would also be understood. We are pretty comfortable with new words in our rapidly changing culture. Weather it is from an new song, or a new app on the computer, we must learn a new word everyday and use it the same day. We sometimes just hear the context and do not ask for further explanation. If you used this in a sentence a person would catch your meaning (weather it is to replace the word young adult - teenager around 18 -22 or whatever - we would get it. It reminds me of the word tween. A tween is 10-12 (before a teen.)