No biggie. I got ready for this in minutes after hearing about it.

#!/usr/bin/env fish
read -P "Are you old enough?  (yes/no)  " input
if test "$input" = "yes" -o "$input" = "Yes"
echo "Proceeding..."
else
echo "You are not old enough.  Exiting." 
exit 1
end

… What? … Why are you all looking at me like that?