There is no magic age when a child should start to learn to swallow a pill. I have seen 4 year olds that can swallow anything and some adults that still can’t swallow the smallest pill. Whenever you are ready to start teaching your child (or yourself) here are two websites the get you started.
http://www.aboutourkids.org/articles/pill_swallowing_made_easy
This website is more detailed. It includes videos and varying positions for those who are struggling.
Dr. K Sun's tidbits 