SHERMAN OAKS, Calif. — Joan Graves, 77, has seen enough sex on screen for five lifetimes. New and inventive ways to kill people? Don’t get her started. She has spent decades assessing off-color humor, deciding what constitutes glamorized smoking and counting instances of the F-word.

Only once, she said, has a horror movie been so violent that one of her employees lost consciousness. Paramedics arrived and hauled the staffer away on a stretcher.

Graves is Hollywood’s ratings czarina. For 30 years, she has watched movies — at least 12,500, she figures — and assigned grades of G to NC-17 so parents can make decisions about what is appropriate viewing for children. For 18 of those years Graves has served as the ratings system’s chairwoman, sparring with boundary-pushing filmmakers who call her too prudish, and, at the same time, defending her process to activists and parents who deem her grades too permissive.

But her reign is ending. The Motion Picture Association of America said on Nov. 15 that Graves would retire in the coming months, to be succeeded by Kelly McMahon, 45, an M.P.A.A. lawyer with a 7-year-old son. “I decided it was time, if only because it doesn’t look good to have a granny in charge,” Graves said in her no-nonsense way. “I can tell you honestly, though, I still love movies. That has never gone away.”