older or elder?
«I was sitting in an arm-chair and watching my sister Alison. She was getting ready for a party. Alison was 15 years __________________(OLD) than me and she always treated me like a child.»
I am sure that's older.
Is elder and older interchangeable?

Thanks in advance.

Question

older or elder?
«I was sitting in an arm-chair and watching my sister Alison. She was getting ready for a party. Alison was 15 years __________________(OLD) than me and she always treated me like a child.»
I am sure that's older.
 Is elder and older interchangeable?

Thanks in advance.

Answer

You need to use "older" in this example. Elder is used when introducing family members. My elder sister. Maria, lives in Toronto.

Answer

Graham is right:

the general form is "older" - he is older than me (never elder), the older teacher said to the younger one etc.

elder is a special form reserved for a few specific cases: as an adjective with family members (my elder sister) and as a noun meaning the ones older than us: our elders, like Robert said.

There is also the derived adjective elderly: an elderly gentleman

Answer

I think no, because you can't put an adjective in the place of a noun, I believe "more old" is a better synonym; I am just an advance English student though.

Answer

They are not interchangeable all the time, but they sometimes are. "Older" is used far more than "elder". If you want a simple rule, use "elder" when you want a noun and "older" for adjectives.  
"The elder spoke with authority"
"My older brother moved to Holland for a while"
In your example "Alison was 15 years older than me" to use elder there you would say "Alison was 15 years my elder" (although this is no longer as common)