We started on ds1's fifth birthday, but he had a good idea of what money was for. He had £20 for his birthday, so I gave him £2 per month, as long as the £20 remained unspent!
He was very happy with the arrangement, and it was wonderful in that he stopped asking for things when out shopping. The answer was, of course, that he had his own money and could save up for things. He always thought very very carefully before spending anything.