I'm thinking ahead to this, myself.
I'm 28, and am testing for my 2nd black this year - My first son just turned 1 on Monday.
Everybody keeps asking if/when I'll start him in Karate.
I'm torn - I know that he'll be watching me do Karate as he grows up, and will want to mimic me, also, he seems to be fairly coordinated so far, so I hope he does well. Soon he'll be on the back of a horse, and learning to ride as he grows, because it will be more natural for him. The motions of karate may come to him more naturally if he starts younger (say 4 or 5), but he won't understand what they mean. But, what's wrong with a good grounding in body awareness?
On the other hand, I'm kind of partial to making him wait, just to build interest. I'm afraid that he may get into Karate because "it's the thing to do", and take the training for granted, and lose interest. If I make him wait for a couple of years, he'll be watching me, interest will grow, and when I finally allow him to start, he'll be more appreciative of karate.
I'm also debating on when to start him with sword training, as well. My training partner has a 2-year old, and I imagine that they will have many bouts as they grow up together.