Can a young voice have physical causes?
In general, a grown up man will have a voice with a lower pitch than when he was an adolescent (Cleveland Clinic). The pitch height basically depends on the length of the vocal cords. According to Wikipedia:


  Adult male voices are usually lower-pitched and have larger folds. The
  male vocal folds...are between 17 mm and 25 mm in length. The female
  vocal folds are between 12.5 mm and 17.5 mm in length.


So, the shorter the cords, the higher the pitch. There are other anatomical structures in the throat, mouth and nose that may contribute to more adolescent-like voice. To know what exactly is the cause in a particular man, an examination by an ear, nose and throat (ENT) specialist would be probably required. Additionally, a voice specialist could have a good insight even without physical examination.  

There can be reasons associated with self-image that make one to consciously or subconsciously insist in speaking with a voice unusual for his age. Sometimes you can see this in singing competitions, when a young man introduces himself with a child-like voice but then sings as a grown-up man.