If you have an older sister, you can also have an elder sister. However, if you have an older house, you don’t also have an elder house. We’ll talk about why in a bit.
As to why we’re even talking about “older” and “elder,” a listener recently asked us to settle a debate.
“[My friend] has three sons. He talked about his oldest son. I corrected him – [his oldest son] would be the ‘eldest.’ The friend answered, “Only when there are two in a relationship, is the older one the ‘eldest.’ Since there are three [sons], the oldest one is the ‘oldest.’”
So who’s right? Let’s look at some history first.
“Older/oldest” and “elder/eldest” go back to the same root. They are both comparatives and superlatives of “old.”
Over time, a distinction started to pop up. In current usage, “elder/eldest” is used only in reference to people, whereas “older/oldest” can be used with both people and things. That’s why you might talk about an eldest sibling but not an eldest house.
It’s true that early on, “elder” did have a meaning of being the senior of two people, but that meaning is now archaic. Therefore, there’s no need to worry about how many people are being compared when trying to decide between “older” and “elder.”
If you’re wondering where words like “elders” and “elderly” fit into all of this, listen to the audio above. You’ll also hear why Professor Anne Curzan thinks “elderly” might be a factor in eventual meaning changes with “elder” and “eldest.”