This code is a mess. C doesn't have a bool data type. True and False are actually an integer with the values 1 or 0 respectively.
#include <stdio.h>
void main( int argc, char* argv[] )
{
  int isteenager = 0;
  int kidage = 13;
  if( ( kidage >= 13 ) && ( kidage <= 19 ) )
        isteenager = 1;
  if( isteenager )
       printf( "teen\n" );
  else
        printf( "not teen\n" );
}