meaning - Proper use of "old brother" and "young brother" compared to "older brother" and "younger brother"

For my entire life, I have been using "older brother" and "younger brother" whenever I write a story in English. I know that "older brother" and "younger brother" are applied to blood-related persons. But what about "old brother" and "young brother"?

I recently read some translated Chinese web-novels that use "old brother" and "young brother" to refer to non-blood related persons. So I started wondering, could "old brother" and "young brother" also be applied to blood-related persons in actual English? For example, instead of using "My older brother and I always go out on a walk in the morning", can we use "My old brother and I always go out on a walk in the morning"?

I read a forum post that *said we use "my old brother" when we refer to our only brother (who is older than ourselves): http://forum.thefreedictionary.com/postst86814_my-younger-brother.aspx
(< scroll down to post by Romany.)

Once upon a time...when Drago and I were educated, there was a "rule"
which only allowed one interpretation: "Younger" indicated that there
were TWO brothers (One is young, the other is younger). "Youngest"
gave no indication of how many brothers one had. And if one only had
one brother, and he was younger than the speaker then one said "My
young brother" - no superlative used.

Like many of the rules which were falsely attached to English, this
one is no longer applicable except in academic writing. But it did
help to clarify the situation, so I still use it - mainly because it

clarifies things in my own head.

In that case, wouldn't using "my old brother" when describing blood related person(s) be correct, or at least acceptable?

If yes, in that context, does "my old brother" mean that my brother is very old (as in 50+), or is it just the same as "my older brother" (can be any age)? Is there a historical precedent of using such terms during a certain era? Or, do we only use them when we consider the cultural differences? Or, can we also use them in the literary writing? Or, how about in the day-to-day conversations as well?

Lastly, in what other situations can we use "old brother"?

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• = the original forum post by Romany used "young brother" instead of "old brother"