Mine all started from when they were 7. Seemed a good time as they were old enough / good enough at maths to understand the concept tht different coines were different values, and could do the maths over how much to save if they wanted to save for something, how long it would take, and also if they spent some of it, how much would be left, etc.
Tooth fairy started when teeth first fell out. But, payment much lower than elsewhere. I explained tooth fairies are feudal and hereditary, generally going down on the mother's side. Ours, if ancient scots and saving thrifty ancestry, only gave x p per tooth. Daddy would match fund it. But much lower even so than the sassenach plutocrats elsewhere in the class. They had different tooth fairies with different traditions.
Pocket money started at 6, I think. Extra money could be earned for chores, extra reading etc.