I don't know how you would ban kids from buying stuff, that's really just not feasible. It would also make things like gift cards, something typically bought for and by kids, a bit redundant.
As for putting limits on: if I, an adult, couldn't buy stuff until tomorrow because of some limit enforced because of idiot kids, I would be pretty P/O'ed.
At best digital storefronts (all of them, not just gaming) should make it more difficult than just throwing in some card details and start spending money. I dunno, even if it requires some sort of bank or even phone verification that you're not some annoying little ankle biter (or a whole list of 'people that aren't you'). Sure, it would be an inconvenience the first time you used a store, but it's a process you shouldn't have to do more than once per store if you create an account, it being your own damn fault if they figure out your awful login information...Would also aid in general theft prevention if the thief had to go through more effort than just slapping in the details literally printed on the card they've presumably stole alongside some easily accessible publicly available information such as address details.
It's honestly pretty ridiculous how easy it is for somebody to start using your card online. IRL we have pin numbers as well as the option to ask for a secondary form of ID. Online I can literally start spending money in seconds just with the info written on the card itself.