boyishness       
noun [ U ] uk/ˈbɔɪ.ɪʃ.nəs/ us/ˈbɔɪ.ɪʃ.nəs/
 男孩子气
the quality of being like a boy
  The actor radiates eager boyishness. 这个演员身上散发着一种孩子气的急切感。 
 He has a boyishness that makes him look much younger than his 48 years.  他身上有一种孩子气,这让他看起来比48岁要年轻得多。 
 - More examples
- He has a certain charming boyishness.
- She brought out the boyishness in Michael.
- Audrey Hepburn had short hair and nobody ever accused her of boyishness.
See: boyish