Confused About Retro

So I've joined a couple of retro gaming groups on Facebook recently and thanks to their stream of delightful posts now filling up my feed it got me wondering, just what constitutes a "retro" game? 

When I think about retro I think about stuff that looks like this 

This 

Or this 

Hell, I guess at this point you could even consider PS2 kind of retro although I hate to admit that because it makes me very much aware of just how old I'm getting.  

What I found interesting is when a post came up saying "What retro RPG would you like a remake of".  The usual slew of SNES games and things like that came up but then one guy came along and said "Lost Odyssey".  This made me raise an eyebrow because that game is on X Box 360, surely that can't be retro, I mean look at it

At first glance it doesn't exactly scream retro does it? For arguments sake I decided to ask my wife how old something has to be for it to be considered retro to her.  She said that anything older than 10 years she would consider retro.  This would place Lost Odyssey, a game released in 2007, in the retro category.

However I also did a Google search and found a forum where a number of users agreed that anything older than 2 console generations is retro, which would then place Lost Odyssey in the category of modern game.  

The lack of consensus presents an interesting topic for debate but becomes a real pain in the butt when it comes to things like streaming on Twitch.  For example I'm currently streaming Ys Origin which was released in 2006.  Now streaming under "Retro" would be preferable because more people would be watching there rather than the individual game.  But Ys Origin isn't a game I'd consider retro really and I wouldn't want to piss off Twitch admin or viewers by streaming in the wrong category.  

To me, retro is pre-PS2/GameCube/XBox is retro and that era is old but old enough quite yet to be considered retro.  I'm always interested to hear other people's opinions on it though.  Like I said before I think I don't want those systems to be retro because it makes me feel old but I'm sure to some younger gamers those consoles are ancient history.