A senior developer is such when they know not just how and why (through experience), but can teach and explain to someone else, and have them understand to a reasonable degree.
I can recall in my early days, a more experienced developer who was paired up with me and another junior colleague, that her job was not necessarily to try and teach us about all possible experience and gained knowledge - we would both have to gain our own badges and scars, and learn from the experiences. She would be there to direct generally, help us back onto the main path if we wandered too far, help with some of the larger leaps and jumps when we were ready, and to of course point out obvious pitfalls and perhaps some less obvious ones too.