What are your ideas to charge them? I thought in paying with the credit in their cell phones, which teens usually have. But I should talking with phone companies to be able to do that.
Have you got any idea?
By the way, this would be a good hole to fill in the internet, so use it as a startup idea if you want to
If you plan on offering the application via the iTunes App Store or Google Play, then you can have them sign in and purchase "credits" via in-app purchases. This way, they can use iTunes gift cards. Plus, which teenager doesn't have iTunes credit?
Edit: In the case of Pay-by-SMS, the parents would find out either way when they see the bill. So, they might as well just use their (the parents') credit cards instead of allowing the teenager to charge it to the monthly phone bill. In this case (and other cases where the parent(s) are aware of and approve the service), you might just want to include credit card payments (via Stripe).
I despise Paypal, but you can always try that.
And because you pay them a commission for each sale, you're not losing money if you offer these options and they aren't used.
Edit 2: Zaypay IS AVAILABLE in Argentina (http://www.zaypay.com/outpayment).
That being said - people who don't have credit cards a lot of the time have parents/guardians who do. You could try that?