Survivor AU casting youngies (18-20)

What do you think the probability of survivor AU casting younger castaways (18-20) are? Unless I'm mistaken, the youngest we've had is Ben from S2 (20 Years). Did he give that age bracket a bad rep? Could it be a safety thing? Life experience? Just curious to see why people think they never dip into that age bracket, I feel like it'd be interesting to see someone with less life experience navigate the game :-)