you have a small error in the code.
Code:
#include <iostream.h>
int main()				//Most important part of the program!
{
  int age;				//Need a variable...
  cout<<"Please input your age: ";	//Asks for age
  cin>>age;				//The input is put in age
  if(age<100)				//If the age is less than 100
  {
  cout<<"You are pretty young!";     //Just to show it works
  }

  else if(age==100)		//I use else just to show an example 
  {
     cout<<"You are old";		//Just to show you it works...
  }
  else
  {      < you have an opening brace here 
    cout<<"You are really old";	//Executed if no other statement is executed
    < but no closeing brace here 
   cin.get();
   cin.get();
   return 0;
   }
may i also sugest that if compiler 4 is dev-c++ 4, get the new dev-cpp 5. a cupple of things are different in the codeing.
not all headers need .h at the end eg. #include <iostream.h> becomes #include<iostream>
and you will need to include using namespace std; then when useing the iostream.