The eldest person in a group is the one who was born first — the one who has been alive the longest. English likes this word for brothers and sisters: the eldest daughter, his eldest son, the eldest of five. You can also say oldest, and many people do, for family and for everything else. Though actually the older, slightly more careful habit is eldest for people in a family and oldest for things and for mixed groups (the oldest building, the oldest student in the class). You almost never use eldest for a house or a book. It is not a verb. You usually say “the eldest” or “the eldest child,” not “an eldest” on its own. It is the “most old” form of old, sitting beside older and elder.