ESRB rating
NetHack is a non-commercial game with no ESRB rating, but be warned: it contains sex, drugs, and rock and rolling boulder traps.
The rating it might attain depends on the quality of your imagination: on the surface, it has no graphic violence (indeed no graphic anything), but gameplay generally involves slaughter on a massive scale, murder, theft, genocide, the aforementioned sex and hallucinogenic drug use, and occult rituals.
Note that ESRB ratings have nothing to do with ESR, a former member of the DevTeam.
