There's a lot of baloney and a lot of controversy and even some interesting science and debate about bodybuilding, but the bottom line is utter simplicity: lift the weight; do it the right way and you'll get stronger as your muscles get bigger.
Bodybuilders have regular holy wars over what the "right" way is. I'm just going to tell you what I've been doing; some people may think what I do is all wrong, and they could even be right. Maybe I would get better results doing something a different way. But at least 95% of what you can find in print or on the net about bodybuilding refers to young bodies, and I'm not young, no matter how you slice it.
I think there's probably an approach to bodybuilding that suits older people optimally, but there's not much real science in this area, and as far as I know, no one is specializing in it. There's a real need for sound, reliable information tailored to bodies that are no longer young. My experience indicates that weight training for older people is very different from training for much younger people. For one thing, the risk of injury is much greater.
I've tried to discover something reliable to guide me. For instance, I had the impression that older people would be wasting their time doing bodybuilding, other than achieving minor increases in strength. I've discovered that's quite wrong. Nevertheless, there are limitations on what you can expect when you're past 60.
I did find a lot of help, however, in certain books. I was inspired by Bob Paris's "Beyond Built," because Paris has a very symmetrical, almost elegant body (for a bodybuilder). Because Arnold is Arnold, and because his book, "Encyclopedia of Modern Bodybuilding," shows the form of nearly all exercises, I found it very helpful. Also very useful, once I had quit floundering around quite so much, was "Kinesiology of Exercise," by Michael Yessis, because it gives considerable detail on how specific movements affect various body parts.
Almost all books have a major flaw, from the point of view of older enthusiasts: they're addressed to people with inherently stronger skeletal and muscular systems, in terms the body's responses to abuse or too much stress. Younger people tend to heal faster and better from injuries, especially micro-injuries produced by stressing a group of muscles to total failure (inability to do the lift even one more time).
But I was to learn that failure comes in more than one form, and that an injury can amount to a very severe setback.