The crossword solution for the term 'Grownup Kid' is GOAT, which is a four-letter word. GOAT stands for Greatest Of All Time and is often used to describe someone who is exceptionally talented or successful in their field. It is also used to refer to someone who has achieved a level of success that is difficult to surpass. In this context, Grownup Kid could refer to someone who has achieved a level of success that is difficult to surpass despite their young age.