Speaking from the Site Admin perspective, I consider anything that saw it's first release more than 5 years ago from the time you're posting to be a valid topic for conversation in the Retro Roundup forum.
The cut off is completely subjective, I know, but historically a console generation typically runs around 5 to 7 years before the succeeding generation begins to be released. A specific console in a generation may last longer, like the Xbox 360 with a run of 10 years that's still not over, but it's not unusual for a console to continue to see releases after the successor is released.
All that said, there's no such thing as a faus pax or getting in trouble when it comes to posting about a game in this forum. The 5 year mark is a general guide, not a rule. If a topic turns out to be out of place, we can easily move it.
At that rate I wonder if Phantasy Star Online 2 will be considered retro by the time it sees a western release. its been out in Japan for years already.
At this point, I'd assume there will be no western release. Sad, but that seems to be the way the cookie crumbled.