Is it too late to start strength training in your fifties or sixties?
No, and this is the age group with the most to gain. Muscle mass and strength decline measurably from around the fourth decade, and that loss drives most of what people think of as ageing — difficulty with stairs, falls, loss of independence. Resistance training reverses a meaningful portion of it at any age, with studies showing significant strength gains in people in their seventies and eighties. Start with supervision if possible, prioritise technique over load, and build slowly. The relevant risk is not training at this age, it is the decline that happens without it.
Answers
My mother started at 58 with resistance bands. Two years on she carries her own shopping again.
The framing that not training is the risk, rather than training, is the one that convinced my father at 62.