For me, Retro is defined by the time. The further back it was made, the more retro it is.
PS2, X-Box and Game Cube are a bit Retro, and anything before the 16-bit era (Mega Drive, SNES) is fully Retro.
On the other hand, the Mega Drive and SNES are a bit Modern. Anything after the PS2, X-Box and Game Cube is fully Modern.
However, if somebody were to produce a NES game/demo today, it would be a modern creation on retro hardware.