At the kid's birthday. As he turned three, four, five? The rule is to light the candles on the birthday cake or the cake - by the age of the child. This gives the holiday a special solemnity, and all guests look forward to: when will you light the candles?
In preparation for the holiday and discussing with your child the different moments of celebration, tell him that every year candles will be more and more light and heat. So he, growing up, going to do more good things, more to bring people joy and light.