In the Netherlands, sex work is a legal profession. In the city of Utrecht, you need to be at least 21 to work or to request a permit for working on the street (Europalaan). Also, you’re required to have a valid residence permit (not a tourist visa), and work voluntarily at a legal workplace.
There are different places to work in the Netherlands: window prostitution, working on the street (red light district), clubs and private clubs, via an escort agency, online contact and from one’s (own) home. No matter where you work, always make arrangements about money and services beforehand.