3 years ago I was working at an Elementary school and I asked the kids "Do you know what Nintendo is?" They stared at me for a while with blank faces (because to them it's a brand, not a system). Then I said, you know.. the old Nintendo? One kid yelled, "Oh I know! You mean Nintendo 64!"

*edit* also, when Mega Man collection came out I saw an 11 year old playing it at a kiosk at gamestop. He was watching his brother play who said he hated it. But his brother responded, "How could you hate it! Mega Man I a classic!", then the little kid responded, "I don't care, it still sucks!"
I would consider Nintendo retro, If it's before nintendo it's still retro only if it was in the arcade (not consoles.. that's too old).
According to google: Retro is a culturally outdated or aged style, trend, mode, or fashion, from the overall postmodern past, but have since that time become functionally or superficially the norm once again
To me, any console before nintendo has not become functional or the norm again. Wii and these virtual consoles has made anything as old as nintendo "cool" again (the exception being good older arcade games).
I think any home console before Nintendo isn't even played by most kids

. I played atari after getting Nintendo, I didn't understand what to do at all as a kid. NES games and beyond are still easy to pick up, colecovision, atari, and all that stuff still confuses me sometimes! So how do I capture the cow? So that block is a knight and that duck/seahorse is a dragon? and how the hell do I throw the basket ball!! and why are there so many buttons! Most atari games were in the arcade anyways, so you can play a better version in those classic collections.