They are written for 8-12 year olds, but we started listening to them in the car when my oldest son was almost 4. We borrowed disc sets at the library for trips, and he would listen for an hour at a time. The character-based series seemed a little too old for him at the time, but the Bible stories are great even for little bits.
Flipping through the free Focus on the Family magazine, I noticed an article on Whit's End. It's the Adventures in Odyssey website. It has a listing of radio program times, games for kids (the ones we played were too hard for my 6yr old), and printable activities.
But I was most excited to see that you can listen free to recent broadcasts online! We listened to one show the other day, and my son has already asked twice to listen again.
I LOVE current media that teaches Bible truths!