What does it take to become a senior developer? Years of experience, of course, but also:
...quality building and debugging skills (can you visualise the problem and the solution before writing a single line of code?)
…solid conceptual knowledge (can you fuel a conversation at the architecture level, rather than at the low implementation level?)
…and, most of all, not require handholding (can you be trusted on your own with little oversight, or even to mentor junior co-devs?)