Held (usually) 3rd Thursday of March. Register Sep-Dec, pay $20 (includes a T-shirt&stuff). They have 6 test papers based on grade.
1-2
3-4
5-6
7-8
9-10
11-12
Anyone can enter at (or above?) their grade. It's individual. The main thing is, is there a test center near you.
http://www.mathkangaroo.org/mk/centers.html
Link currently displays test centers for Mar 2014 contest.