13 years if you start at kindergarten. I'm not sure what age/grade they start with standardized tests though, which is what I think you'd have to base the kids performance on.
Casper, we are going to have to agree to disagree here. I don't think they'd see extra as a bonus, it's become expected just like fly said, whenever people get a raise they raise their expenses because it's expected income, it's not going to ever be treated as just a bonus except maybe the first few times they get it.
And I still maintain it'll prevent people from having kids just for a welfare payment (if that's actually a problem) because forcing them to be a decent parent might turn them off to the idea.