Junior designers are those with some training and (hopefully) some talent. They've gone to design school or done a handful of professional projects. They can create deliverables, although not without supervision. They can often explain design decisions, and can handle problems of moderate complexity. Junior designers should always be paired with a more senior designer.
Senior designers are those with the magic 10,000 hours of experience that make one a master. For those who are math challenged, that's 250 40-hour work weeks, or over five years being a designer. In my experience, this is more like seven to 10 years of being a working designer to reach this level of mastery, where one is able to tackle complex problems in a variety of domains.
I would love to see this standard applied — there are many people that have a false idea of where they are in the industry (including myself ;) ). Read the complete article to see Dan Saffer’s thoughts.