#include <iostream>
#include <limits>
int main()
{
  int age = 0;
  while ((std::cout << "How old are you? ")
         && !(std::cin >> age)) {
    std::cout << "That's not a number; ";
    std::cin.clear();
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
  }
  std::cout << "You are " << age << " years old\n";
  // ...
}