At what age/stage should my child be at before I start using a child seat?
The Bicycle Helmet Safety Institute, the US Consumer Product Safety Commission and the American Academy of Pediatrics agree that:


  ...children under 1 year of age should not be on bicycles. Children are just learning to sit unsupported at about 9 months of age. Until this age, infants have not developed sufficient bone mass and muscle tone to enable them to sit unsupported with their backs straight. Pediatricians advise against having infants sitting in a slumped or curled position for prolonged periods. This position may even be exacerbated by the added weight of a bicycle helmet on the infant’s head.  


In some US jurisdictions it is illegal for children under the age of 1 to be passengers on a bicycles as well.