I had this discussion (read argument) back on reddit a while back. And it's about 15 to 20 years or at the time people were calling PS1/N64 Retro during the PS3 era.
By my math systems like the PS3 and Xbox 360 are now "retro", at least for some kids. I defiantly feel dated when some kid shows up on a PSP thread going on about their Dad's old PSP. And it's weird to think that the Vita and 3DS are about to fall into that camp.