Elder and older both mean "advanced age," so I see where the confusion is. In the example phrase you have listed, elder and older would be able to fit interchangeably. A key difference is that elder is used only with people while older can be used for anything (animals, objects, etc.) Ex: "The building is getting older." You can't convert this to "the building is getting elder." Elder is viewed as a more polite, formal way to say old, generally. Someone who is very old and wise would be called an elder. On that note, elder can be used as a noun while older can't. Also, elder can't be used to compare things like older can. Ex: She is an elder. You can't convert this to "she is an older." Ex: She is older than him. You can't convert this to "she is elder than him." If you have any other specific sentences/phrases you'd want to review about the usage of older vs. elder I'd be happy to help. This is just a quick explanation.

It's a bit confusing. elder and older are synonyms in the context you have given but they rather different words. You can have old, older and oldest but there is no adjective "eld" Although "eld" is an archaic noun that I have never heard or seen used. elder can also be a noun meaning a senior or important person in a group. You can have tribal elders or church elders.

I have an older brother. (Older is a adjective that describes him like tall, smart, rich etc) He is my elder brother. (‘elder brother’ almost goes together like a title. ‘older brother’ would still be more common though.) An elder is also a type of tree/bush.