ESRB ratings are used in the USA.

EC Early Childhood No Kirby/Zelda Games
E Everyone 5+

All Kirby games except for Kirby's Return to Dream Land

Most Zelda games

E10+ Everyone 10+ Kirby's Return to Dream Land, Spirit Tracks, and Skyward Sword
T Teen (13+) Twilight Princess, Link's Crossbow Training
M Mature (17+) No Kirby/Zelda Games
AO Adults Only No Kirby/Zelda Games