Had this conversation with a 26 year old male. He has finished a degree at Uni and told me I was wrong when I said you cannot call your younger brother your older brother. He is only older son to your mother who would refer to her three sons as "my youngest son, my older/elder son and my oldest/eldest son. He was so passionate about this (my three sons also told me I was wrong) I had to stop but at 47 I am wondering what is correct.
To take it from from a linguistic boffin's standpoint, I guess that "older" is a deictic word: its meaning varies depending on usage. There's a handy illustration at Wikipedia's page for deixis. When you say "my older brother" you put yourself in the deictic centre. Your listener then should figure out the position of your brother relative to that centre.
As WS2 rightfully pointed out, you may with a degree of certainty say "he's my older brother" to an acquaintance who knows that you're the oldest of the flock, but not to just any John Doe.
That's because deictic words require contextual information to be fully understood, and your acquaintance will know that while you're at the deictic centre there's no way for you brother to be at the.. er.. deictic "above", because there's a.. hm.. "deictic ceiling" of the fact of your being the oldest.
It would depend to whom he was talking. If it was someone who knew the family well presumably he would mention his brothers by name. If it was to someone who just happened to know he was the eldest, 'older brother' might just scrape in as acceptable speech. But if it was to someone who didn't know him or his family from Adam, for clarity's sake he would need to say 'The older of my two younger brothers'.
WS2 beat me to it, but here's another possible explanation...
In this case--you're both right. One can concoct a scenario where it is possible for him to justify calling his clearly younger brother an 'older' brother:
In the meaning you imply in your question, (and that most people expect when siblings use "old" or "young" as a comparative adjective,) older refers to the fact that one sibling has lived longer. So the sibling born first is old*est*, and old*er* than the next, and so on. The sibling born last is young*est*, young*er* than the next, and so on.
There is another meaning for older. In essence, older simply means 'has more age than', and so it is important to specify the frame of reference. What your son could be claiming (although he is really just being a pain) is that if he knew his brother a certain point in time in the past, and he still knows him now, then your youngest is actually, older.
He is "older" in that he has now accumulated more age than his previous self.
Here's one for you that ensued in a big argument. I have four older brothers. I referred to my next eldest, the third eldest as my youngest brother. I was told that I was dead wrong because he was not younger than me. But I am not wrong. It dosn't matter how you cut it he is still my youngest brother. I never said he was my younger brother, only the youngest of my brothers. Of my four brothers as a group, he is the youngest. Can anyone out there logically contest this?
If Abe, Ben, Cindi, Dave (listed young to old, with Abe, Ben, and Dave being male) are the siblings in a family, then Cindi can unambiguously refer to Abe, Ben, and Dave as follows:
• Abe: “my youngest brother” or “my younger younger brother”
• Ben: “my older younger brother”
• Dave: “my older brother”.
The phrase “my younger younger brother” is most likely to be used only facetiously or sententiously, since phrases “my youngest brother” (when there are more than two other brothers) or “the younger of my brothers” (when there are two other brothers) typically serve better. But phrases like “my youngest younger brother”, “my youngest older brother”, “my older younger brother”, “my younger older brother”, and “my oldest younger brother” all serve useful purposes in different cases.
If Cindi refers to Dave as “my oldest brother” rather than “my older brother”, the listener isn't told whether Dave is older vs younger than Cindi; some listeners might assume Cindi is older than all her brothers, some may assume she is younger, and some will realize they don't know.
It seems to me that it would be correct, provided you've already excluded yourself from the group you're referring to. Like others who've posted here, I agree that it could lead to some misunderstandings and should be refrained from. When I grew up as an only child, I ran across the same sort of situation with the large number of cousins in my family.
Because he made it clear that he was the oldest child, he had already defined boundaries which excluded him from the group of "my younger brothers." However, this could make sentences confusing.
"I'm the oldest of three brothers. We all run a deli. My youngest brother works in the back while my older brother takes orders at the counter."
Saying "the older one" would work better than saying "my older brother." However I believe the statement can still be understood. We could also employ the word elder here to avoid some of the confusion in using older/younger together.
"I'm the oldest of three brothers. My youngest brother works in the back while my elder, younger brother takes orders at the counter."
"The older of my brothers." Comes across as a grammatically awkward statement. It doesn't sound right when I attempt to annunciate it. I could be wrong, since I don't have a PHD and am just an ESL teacher, but it doesn't sound smooth.
I have also heard people differentiate which younger or older sibling they were talking about by using bigger and smaller. "My big, younger brother ate the cake."
"My siblings are not so nice to each other. My sister is still very young - it is horrible that she is always being bullied by my older brother. "
First I give a context in which I am comparing my siblings among each other - making the interpretation of 'older' as 'older than me' less likely. Furthermore, I talk about my young sister, to contrast her youth to my brothers relative oldness. Also I talk about my sister instead of my brother, so as to make it more logical to use the phrase "my older brother" instead of "the older one". I think all these factors should lead one to believe that I am calling the brother older in comparison to my sister, and not to myself.
In these circumstances I wouldn't know any other way to make clear that the relative oldness of my brother is in comparison to my sister, instead of me. Therefore, anyone would be justified in interpreting the sentence the way I meant it.
Let's keep it nice and simple; and say
"My older younger brother."
Everyone will get the meaning of that phrasing.
To say "my older brother" puts you at the centre of the group to which you are applying the comparative adjective, and therefore includes you in the comparison (and therefore implies older than you). To say "the older of my brothers" puts the brothers at the centre of the group, and applies the comparison to the brothers only. There is no implied relationship with you. This ties in nicely with @CopperKettle's comments.
My main point of comment, though, relates to your
He has finished a degree at Uni...
I seriously hope this doesn't factor into your judgement about his credibility to be authoritative in this matter. If so, I've done a PhD, and I agree with you.