And few lines could be thrown away, because IMHO last case will be never executed. And assignment operator is '='; '==' is comparison.
#include <iostream.h> 
void main() 
{ 
int age; 
int myage = 14;
cout<<"plz input your age: "; 
cin>>age; 
if (age<myage)
  cout<<"you are younger than me!"; 
else if(age==myage)
  cout<<"you are the same age as me"; 
else 
  cout<<"you are older than me!"; 
}