I started riding with Ellis when he was pretty young, maybe 4months old. But this was only to get him use to it. I would basically just go very slow around the block. When he was about 8 months we would go a little further but not any serious running until he was a year old. You dont want to put too much stress on their joints while they are still growing.
I have a bike attachment called the k9 jogger. It attaches to my bike seat and keeps him from pulling me off my bike. Even if he sees a squirell he cant pull me over.