Learn English – use of “not on purpose”

You can use the expression "You can do it of your own free will" or "You can do it of your own volition", or simply "It is voluntary", in other words, "It is not compulsory", to describe things that are volitional.

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