At what age is a child obligated to wear a kippa and tzitzis?
The gemorah requires educating a child to wear tzitzis from the time he knows how to wear a tallis (wrap it around properly.) Some say this is five or six years old, some say nine. It is common for children to start wearing a tallis katan after toilet training, but it certainly is not an obligation, but a way of accustomizing the child to do mitzvot. The same is true of a kippah. No special time is mentioned, but it is proper to make a child aware that mitzvot are a natural part of life. There is no obligation to push it, though, and it is more important that the child enjoys the mitzvah and that it makes him feel responsible and grown up.