It's possible to be a citizen of a country without having ever used any resources from it (it's also possible to have dual citizenship, which complicates things

It's possible to be a citizen of a country without having ever used any resources from it (it's also possible to have dual citizenship, which complicates things). Taxation isn't about retroactively collecting money for the resources you used during your childhood, it's about collecting money for the resources you're using today. Children, the unemployed, etc. are sponsored by the rest, they aren't taking a loan on their taxes.