man-child       
noun [ C ]  usually disapproving (also manchild) uk/ˈmænˌtʃaɪld/ us/ˈmænˌtʃaɪld/
 孩子气的男人,长不大的男人
an adult man who does not behave in the calm, serious, or sensible way that you would expect from someone of his age
  Her fiance is portrayed as a feckless man-child. 她的未婚夫被描绘成一个无能的孩子气的男人。 
 When his first child was born, the jealous manchild left home. 
 - More examples
- They described him as a playful, quiet man-child with tremendous drive.
- To call him a man-child gives children a bad name.
- They aren't grown-ups but man-children who are afraid or unable to commit to relationships.