Needless to say, there are many different opinions on what a senior dev is or does.

According to some people on StackExchange a senior developer can:

• handle the entire software development life cycle, end to end
• lead others, or others look to him/her for guidance
• can self manage projects

I think this is a reasonable answer but sure enuogh people added more:

• I would disagree with the second bullet point
• at 22-23 you should not be a senior devloper. (Why exactly?)
• Accountability
• you have had at least one project fail under your leadership
• etc

So what are the key things defining a senior dev in your opinion?
Or, if you prefer the via negativa, what is a "junior" missing to be a senior?