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Like many of the rules which were falsely attached to English, this one is no longer applicable except in academic writing.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"?

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

As mentioned in the comments, it's never standard to use "old brother" or "young brother" in place of "older brother" or "younger brother".

It is not idiomatic in any register or context to use old brother to mean older brother. I read Romany's post and I have never heard of the practice he describes: I think it was a usage invented and used in his family. No one else would understand what he meant, whether in speech, academic writing, literature, or anywhere else. The Chinese web novels are trying to translate a concept that doesn't exist in Western culture, so the language is necessarily going to be awkward.

– Dan Bron

"Older" brother means that he is older than you. If you are 5, and he is 6, then he is older, but not old. If you are 5 and he is 50, then he is both your older brother and old brother. (Also, the word "old" has different meaning to different people.) If you are writing English, don't use "old" for this.

– MikeP

"My old brother" can be used in the sense of "my brother, who is old", and "my young brother" can be used in the sense of "my brother, who is young". However, these phrases are not commonly used. "Brother" in this context could refer to a blood relation, or to any of the other possible meanings of the word.

Generally speaking, one would not use "old brother" at all, except when the intent was to emphasize the age of the individual.

– Hot Licks

Monks are often called brothers, as are communist party and trades union members. So, referring to "a young brother", etc, is not necessarily incorrect. However, for blood relations? I don't think so.

– Mick