Cain was older than Abel.
According to the biblical account in Genesis 4, Cain is explicitly described as the firstborn son of Adam and Eve, followed by Abel as the second son. This makes Cain the elder brother by birth order, with no textual indication of Abel preceding him.

Biblical Birth Order

The Genesis narrative states: "Adam made love to his wife Eve, and she became pregnant and gave birth to Cain... Later she gave birth to his brother Abel".

Cain, as the farmer, and Abel, the shepherd, were adults at the time of their offerings and the murder, suggesting some years between births.

No ages are specified, but Cain's seniority is unambiguous.

Scholarly Interpretations

Many biblical scholars and creationist analyses affirm Cain as older, often estimating he was at least in his 20s or older—potentially over 120—when he killed Abel, based on genealogies like Seth's birth after the murder (Genesis 4:25).

Views vary: some timelines place Cain at 29 and Abel at 22, while others see them as mature elders in a pre-Flood world of long lifespans.

Jewish and Christian traditions consistently uphold Cain's firstborn status.

Forum and Modern Discussions

Online threads, like Reddit's Bible commentary, echo this without contradiction: Cain as older.

A unrelated game context (RomanceClub) flips it fictionally, but biblically, it's unanimous.

TL;DR: Cain was definitively the older brother.

Information gathered from public forums or data available on the internet and portrayed here.