[gaius]

You might expect it, but I've encountered plenty of 23-year-old Senior Network/Software/whatever Engineers with just 1 year of experience. Basically as soon as they'd implemented a single feature, they would consider themselves senior.

Personally I say a senior engineer is one who solves problems by the most appropriate means. Maybe he fixes a "bug", maybe he/she knows enough about the problem domain, the use cases, the dependencies, the history and changes the documentation instead. Maybe he/she tells a junior engineer "figure out how to scale that" and uses experience to know what part of the system really needs to scale. And so on and so on. A senior engineer spends little time coding and lots of time mentoring.