Smith is going bald. He's a nineteen-year-old fireman. He couldn't care less about going bald.(single sentence)
Smith, a nineteen-year-old fireman, couldn't care less about his going bald.
Smith is going bald. He's a nineteen-year-old fireman. He couldn't care less about going bald.(single sentence)
Smith, a nineteen-year-old fireman, couldn't care less about his going bald.