"When" is used with time.
So "earned at a time, when.."
"When/which" can be interchangeably used with "age"
Therefore, "age, in which..."

First of all, on the OG13, it doesn't say, "age, in which" and "a time, when". It says, "an age in which" and "a time when". Please refer to the mentioned questions on my post.

Then, isn't 'an age' also 'a time'? So, if we can say, "an age in which", what's wrong with saying, "a time in which"?

Great question and, actually, the use of "an age" and "a time" describe pretty similar periods of time in those two questions which I think makes the context that much trickier.

I'd look at it this way - "a time" represents exactly that, a time. Whereas "an age" or "an era" represent a time period. And that's the little distinction. It if it's a specific time, that's a "when" situation. But when the intent is a longer time period - an age, an era, etc. - then something can happen in or during that period.

It's a fairly subtle distinction, but the time period can use things like "during" or "in which" whereas the actual time can't. You couldn't say "during Tuesday" but you could say "during Tuesday's staff meeting".

Thank you so much, Brian. Your explanation couldn't get clearer. I just wrote down what you said in my Verbal notebook so that I wouldn't forget it later on. Thanks a lot, again.