What is the difference between "Older" and "Elder" ?
And are they interchangeable ?
The use of elder is restricted to compare human beings, mostly in family relationships. The one who is elder is the one who was born first.
This is my elder sister Betty.
You cannot substitute elder for older at all times.
Elder can only be used for people, when used for things, it is meant as a special figure of speech involving personification, so to be used with extreme care.
You cannot use elder with "than" (even if it is implied, as in sentence b) ):
a) This is my sister Betty, she's older than me. (and not elder)
b) This is my sister Betty, she's older. (and not elder)
- This is my sister Betty she's the elder of us two.
We do not use elder after get or grow:
- You'll understand when you get/grow older.
The restrictions in the use of the superlative eldest are the same. It is not used for things.
- The eldest/oldest of my six children has already left school.
- My eldest/oldest son is in the fifth grade.
He's the oldest in the school.
Elder and eldest are considered formal and some people do not use them at all.
I generally agree with most of the quirky rules in the other answers. However, I'd like to add: in American English, the adjective elder seems practically antiquated. Eldest is still common, e.g. as in "the eldest son". Even the noun elder, i.e. when referring to the older people in a person's life ("their elders"), is common. The adjective elder, however, is generally left out of normal speech and writing in favor of the almost perfectly-synonymous older. I only ever see or hear elder used as an adjective in fantasy where the word's nature as a nearly-forgotten lingual relic makes it seem almost foreign and exotic, lending to a setting's mystery and age.
Elder can be used to indicate someone or something of higher rank or greater influence. The elder member of a group may not always be the oldest member of that group.
Older and elder both imply having greater age than something or someone else.
As stated in the other answers, "elder" means "older" but is usually only used when comparing a group of people who are in some way related. It also comes up in some idioms such as "elder statesman".
There is one slight subtlety: consider the siblings John (15), James (17) and Mary (19). Mary might talk about "my elder brother, James" to mean "My brother James (by the way, I also have another brother who's younger than him)", whereas James might talk about "My older sister", meaning "My sister Mary who is older than me."
Used as an adjective, "elder" is used when comparing the age of two people. The person who was born first is elder. It's used often for siblings, as in, "I have two sons. Charlie is the elder. John is the younger." While this is correct, in every day speech, most people use "older" instead of "elder". "My older son, Charlie, is coming to visit today."
Older can be used to describe things as well as people. "My house is older than yours." Elder cannot be used in this way.
They are almost completely interchangeable.They are basically synonyms. However, 'elder' sounds more formal than 'older'.
The comparative and superlative forms of old are older and oldest.
My sister is older than me.
That's the oldest castle in the city.
I agree with @Michelle, elder generally should not be used to describe things.
"older" is the normal comparative, the use of the variant "elder" is limited to a few cases. Oald has it very clearly: http://www.oxfordlearnersdictionaries.com/definition/english/elder_1?q=elder
After reading everyone's input with great interest and much gratitude, it seems to me that the word "older" is meant to signify that something or someone is more old, whereas the word "elder" does not actually refer to age, but more to experience or understanding.