From the Web-site of the Louvre:

  Admission is free for the following visitors:
  
  Under 18s
  Proof of ID required.
  
  Under 26 year-old residents of the European Economic Area (EU, Norway, Iceland, and Liechtenstein)
  Proof of ID and residency required.

  All visitors
  On the first Friday of the month after 6 p.m. (except in July and August)

(Source: https://www.louvre.fr/en/visit/hours-admission)