Three of mine asked about communion, and we told them after they were baptized. Then we talked about why we get baptized and about being saved, it wasn't until we were comfortable with their profession of faith that we allowed them to be baptized. Mine were 5, 7, 7, and soon my current five year old, who has been asking for months, will be baptized. I think mine were rather young, but they do tend to the gifted side and made it clear to us that they understood what it all meant.