My son knew them all by about the 6th week of Kindergarten, so about 5 1/2. He's now 10 and doing great in school. I'm an Early Childhood Curriculum specialist, I coach teachers and write curriculum, and I feel that this is a skill that's way overrated, and pushed way too hard early on.
I agree that the Leapfrog videos, or for a 4 or 5 year old, the Leapster games are a great way to teach this. I also really like Starfall.