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re: "because that is the mininum age required to work here in the Netherlands."
This has nothing to do with the minimum age required to work.
Upwork freelancers are not employees.
The 18-year-old minimum age requirement has its basis in the minimum age for entering into legal contracts.
A child under the age of 18 is considered a minor and is unable to sign a contract.
So Upwork doesn't want to host a system for freelancers who are too young for the contracts they enter into with clients to be considered legal or legitimate.
What Preston meant is that Upwork is a US corporation and in most US states, 18 is the minimum age to enter into a legally binding contract. Upwork, therefore, has made 18 the minimum age to join, no matter where you live.