It should be important to remember that what is considered "retro" depends on the demographic of the site, and if the average age changes, so will the definition. Already there are kids who consider the PS1 and Dreamcast to be old school.
If I had to define retro, I'd say anything at least 20 years old. That should be a big enough timegap so we won't get people writing about anything that would be considered "modern".