Actually you do, depending on the location (and thus local laws). You definitely don't get an employment contract, and that means that e.g. in the whole EU, you need a business license (self employment or a company); some EU countries allow that from 16 and some (most) from 18; it's illegal to make an invoice if you don't have a license, and Upwork requires you to do so.

You're right that I could've worded that better, that's what I meant with the last part of my comment.