Both sentences are odd. The past perfect is used to describe an action or state that occurred before another action or state in the past.
So, your first sentence, is incorrect. If you are trying to say that he started attending church after completion of his cure, the sentence should read
"He started attending church after he had been cured of injuries that he had suffered in an accident."
The attendance began in the past, but after the cure, which in turn necessarily occurred after the injuries were incurred. Notice the sequence of events. Also notice that "when" is not as informative a choice of word as "after."
If you mean that the attendance started while the cure was taking place, the sentence should read
"He started attending church while he was being cured of injuries he had suffered in an accident."
Now the sequence is different: attendance and cure are concurrent, but followed the accident. Again, "when" is a poor choice of words; "while" would be clearer. But notice that the past perfect is still used about the accident because it preceded the concurrent past events.
As for the second sentence, it is hard to imagine a context where it would be correct. What past event did this discussion precede? You could say,
"People had started discussing my retirement long before I reached forty."
Although you did not ask about it, the "as" in your second example is another poor choice of words. I am not sure whether you mean "because I was in my 30's" or "at the same time as I was in my 30's."