There's no defined age where you can stop building muscle, but at a certain point, you'll do a lifetime PR without realizing it will be your last.
Rip wrote a piece about this for people who have already been training a long time and are getting to the point where training isn't viable. Sully wrote a follow up piece to it as well.
The answer to your question is "we don't know," but even as she can't hit new lifetime PRs, she'll be able to hit local PRs. PRs in her 60s, PRs in her 70s, and so on. There is an olympic lifting record held in the women's division for three consecutive age classes, because the 50-year-old woman refused to stop lifting. She has the 50 to 60 total record, 60 to 70 record, and 70 to 80 record. Her numbers steadily went down, but I'd bet she was more healthy and athletic than the other 80-year-olds she knew.