Maariv website (quoting Yated Neeman) had a nice piece on this. Some people say that it is from age 3. Other suggestions are given.
My kids have very different feelings when it comes to wearing a kippa. One of my boys just loves covering his head. The other takes off his Kippa whenever he can! So I was very happy to hear the answer of the Rav Chayim Kanievsy (from Bnei Brak, son of the steipler). When asked "When should a child wear a kippa?" He answered: "When he stops throwing it off his head!"
Sometimes sensible straight thinking rules!