What's the main difference between "eldest" and "oldest?"

Native speaker’s answer
Rebecca
The difference is what you use each of these for! "Eldest" is used only between a group of people to determine who has the greatest age among them. Whereas "oldest" can be used with people or objects, but not to determine seniority in terms of age. Ex: He's the eldest of five siblings. Ex: Out of all the buildings in town, this one is the oldest!