When is a senior developer senior?

You shouldn't really be hiring senior developers. When taking on a new employee, that individual needs to learn an enormous amount about your workplace, process and products. You wouldn't consider them an expert in your company's doings, so you shouldn't label them as senior.

Although I personally frown upon the senior developer title being thrown around, I feel that advertising both junior and regular developer positions is important - You'll want to hire experienced programmers, but let them come onboard and prove themselves as just a developer (many articles on this topic mention the "intermediate developer", but I'll be mainly referring to the actual job title rather than the skill level).

Though I only have a few years experience to base my opinion on, I feel that my involvement in full-stack development teams and my awarded senior titles has given me valuable insight into the caveats that come with seniority. A senior developer is not only someone that has 5-10 years experience in any given field, but rather someone that has expert experience with the software product they're developing. They should certainly be able to work individually without instruction, provide guidance to those with less experience, communicate effectively to customers and management etc., but this is not as important as knowing the project intimately.

Senior job description

Although a great portion of job offers specifically request senior developers, it's hard to know just how many are employed under that exact title. I expect that most of them are immediately granted the coveted title, and this is what I'm confused about. You're shopping for a smart, experienced individual, which you may just find with your job ad, but you've only just met that person - why would you want them to represent your company as a senior developer?

When external parties come in contact with your company, a lot of the times it'll be through a verbose communication like email. If I were to deal with a senior developer at some company through email, I'd expect that they'd have nothing less than a thorough knowledge of the inner-workings of the product that they're representing.

Hiring a senior developer into an already established team can also bring some avoidable friction. Although the new arrival may have a wealth of experience, they are effectively a novice when it comes to working with that company's platform.

I would suggest hiring a Developer, awarding them a more senior label after they've become an expert in the system(s) they're building. Job ads can be written such that they still appeal to the more experienced applicants whilst not selling the 'senior' title.