Hi orangedonkey,
That's interesting i received exactly the same letter from SIPA, 'impressed by my academic background but too young/not enough professional experience". I am a 23 int'l student, so i know that hurts ! Still i imagine age is not the main criterium for admission (in their stats i think they mention they accepted students from 21 to 42), but if you are young the race is even more competitive because of course you lack the experience others have gained in 4 or 5 years.