OK, this thread reminded me I was missing some good opportunities with my kids.  So last night I brought a target out in the living room and put up a target of a big fat prairie dog.  My year and a half old son was super excited.  

I put his hand above mine on the tiny bow.  And we would draw together.  I'd tell him to let go and I would release after he was clear.  He loved it and kept saying "more".  We shot probably ten times and I made sure to end the session while he still really wanted to keep shooting.  While we were shooting, my five year old daughter quit playing the Wii to come and join us.  We all had a great time and it took less than 15 minutes.  Plant those seeds as soon as possible.

Is my son too young to technically shoot a bow, yes.  He can't pull the bow or coordinate the shot yet, but we have a great time together and he really wants to participate.

All four of my boys were shooting by two. Now they weren't hitting, but if you just let them do their thing and keep it fun praise every shot and don't worry about form, accuracy or any of that until later you'll have him hooked for life. Good luck and enjoy the time together.