There are Gen Z that played PS3/Xbox 360 when they were children, and are now adults out of college with their own children. I think that's a good measure of "retro", and it would mean PS4 will enter retro status in ~2031.
Most modern games could be ported to the PS3 and Xbox 360, keeping gameplay the same but lowering graphical fidelity. The same cannot be said of porting N64 games to the NES. I think a better measure of retro-ness would include the degree to which gameplay experience is influenced by the game's age.
Many in the community, such as the long-running Retronauts podcast, use 10 years as the line when something becomes "retro". By that definition, the PS4 becomes retro later this year.
I'd even argue that any console that is no longer in production can be considered retro; after all, you're then dependent on secondhand consoles and repairs to be able to play things on them; they have become a finite product.
