When our DS was younger, he didn't get charge privileges. When he turned about 13 or so, we started to give them to him, but monitored the folio every other day or so. Now that he's 17, we never see him, and he has charge privileges. We still make sure he's got some cash with him because not everywhere takes the charge, but he's been VERY good about not only using the charge wisely, but also bringing back the receipts. He spends most of his evenings at DQ (has an AP for there) and usually chooses to eat burgers and fries there rather than dine with DH and I (we generally eat at an upscale place, and although he enjoys the food, he doens't like to spend that much time eating--too many things to do!).
I think it just depends on the kid, and that kids maturity level. We've never had a problem, although we also didn't allow him to go places at WDW on his own much (and for limited reasons and amounts of time) until he was 16. So he never really had much of a chance to abuse anything. When he has made a purchase other than food or drink, he's understood that he needed to reimburse me for the souvenier...again, by the time I began allowing him to be away from us and even have that opportunity, he was mature enough to understand it.
Like I said, I guess it depends on your kid. I wouldn't give an 8 year old charge privileges, the chances that they will ever be alone and have to buy something are very slim anyhow. When you feel that your child is mature enough to go to the parks by themselves, they are probably also mature enough to have charge privileges. To me both are at about the same maturity level.
Anne