The only word that I know of (American Heritage Dictionary now recognizes it) without a written vowel is "nth". Don't forget, Anon, that y sometimes serves as a vowel, not just a demi-vowel.

what is the longest word that a 12 year old american can say

Longest Words (45) PNEUMONOULTRAMICROSCOPICSILICOVOLCANOCONIOSIS (also spelled PNEUMONOULTRAMICROSCOPICSILICOVOLCANOKONIOSIS) = a lung disease caused by breathing in particles of siliceous volcanic dust. This is the longest word in any English