Problem 21 (2011 AMC 8)

Students guess that Norb's age is $24$, $28$, $30$, $32$, $36$, $38$, $41$, $44$, $47$, and $49$. Norb says, "At least half of you guessed too low, two of you are off by one, and my age is a prime number." How old is Norb?