It has been my experience that “Senior” in a job title is subjective and varies from company to company.

At my last place of employment, “Senior” was synonymous with Team Lead.  Unless the “Senior” DBA left the company,  no one else could be a “Senior” DBA, even if they met or exceeded the qualifications for “Senior” DBA as written in the HR job description.

At my current place of employment, “Senior” means that you have a very good working knowledge of the product, you have a demonstratable track record of applying that knowledge correctly for a number of years, and that you can give advice on how to improve the system.

I would agree with my current company’s philosophy.  Until a person is to the point where they can regularly give advice on improving the system as opposed to only operating the system, they have not graduated to a Senior level.