Years of experience
Total years across a developer's dated work history. It sets a floor on exposure and predicts actual seniority weakly.
What it tells you about a developer
Use years as a coarse screen and read the work itself for the real answer. A profile with senior-shaped work at a low year count usually marks a fast riser, and the work should be believed over the number.
Years of experience sets a floor on exposure. Adding up dated roles rules out true beginners and then loses predictive power fast. Scope of responsibility, ownership, and trust from other engineers separate senior from mid far better than tenure does.