The ESRB's M (Mature 17+) rating for GTA 6 cites violence, strong language, and nudity. Grand Theft Auto is a crime series told from the perspective of criminals; the content descriptors reflect that directly.
M-rated titles are intended for players 17 and older. In practice: most retailers ask for ID at purchase, and both PlayStation and Xbox family settings can block M-rated content on a child's account. The rating isn't a warning label so much as an audience statement — this game is built for adults.
Every mainline GTA has been rated M, and GTA 6 is not a departure. If you're weighing whether it's appropriate for a younger player, the honest answer is that it is not designed for children, and the console-level parental controls exist precisely for this situation. We're not here to tell you what to allow in your home — just to make sure the facts are clear before launch day, not after.
Age ratings vary by country, and some regional ratings may still be listed as pending. The M rating discussed here is the North American ESRB rating referenced through Sony's official channels.