Why can't we just go the Japan route? Not even person under 21 is a reckless ten year old who drops thousands on Angry birds 32.
For anyone not familiar, Japan basically places a limit depending on the game. For example, a game will have 5000 'points' that you can use to buy microtransactions with. Once you spend the equivalent of X points (typically hovering around equal values), then you have to wait until the beginning of the next month.
I think it'd be a better solution instead of an outright banning. the labels however aren't too shabby of an idea - it'll help the uneducated know what games they're getting for themselves and others