S.H.
I believe they are old enough to understand the value of money, and working for it. I wouldn't buy them anything expensive (game systems, bikes, TVs, etc.) until they have learned to take care of them.
Do the boys have chores that they are responsible for? Whether they already have some or not, maybe some chores could be "extra" on their list. If they do these extra chores, they earn a quarter or 50 cents, etc. per chore. Let them keep the money in their own piggy bank or something. When they want something from the store, tell them they have to buy it themselves with their money. They will realize that you have to work to earn the money, so whatever you buy with it has to be what you really want, and you have to take care of it or you will lose it!