So, when can I expect said pains?
It's different for everyone, but as was pointed out upthread, most people agree that it's in your 40's when you start feeling all the effects of what you did to your body when you were younger, and start to notice that you're just not as young, quick, and sharp as you used to be.
Most people who never wore glasses before end up at least getting reading glasses in their 40's. The 40's is also when you first really notice that it's tougher to keep the pounds off; you can't eat like you used to and just metabolize it anymore, because your metabolism is slowing down. That's why people in general start getting fat as they get older (that and the fact that most can afford more and better food, but don't shake the habit of eating everything in sight, given the chance). That kind of thing.
Plus, it's around now that if you had even a minor injury when you were younger, banged your knee on something, for example, it really starts to bug you now. Sometimes.
But remember, people didn't even used to
live to 40 most of the time. George Carlin said that cancer, liver disease, heart failure, high cholesterol, all those horrible diseases... we used to have a cure for them. It was called "death". Modern science has enabled us to live well past the age we used to, but evolution hasn't caught up yet. So we live in bodies where things are starting to break down and not work right anymore. But it still beats the alternative.