It's hard to differentiate what is and isn't retro sometimes.
Some people considered the PS2, GCN and Xbox (plus DC) "retro" this past generation, when the PS3 & 360 were around.
Chances are, when the PS4 & XB1 are in 'full swing', PS2 era will probably be considered retro...
I think counting 2 gens back probably qualifies a machine as being retro:
2600, C64 etc (8-bit) when SAT, PS1 etc (32-bit) took hold in 95ish
SNES, MD etc (16-bit) when DC, PS2 etc (128-bit) took hold in 2000ish
SAT, PS1 etc (32-bit) when PS3 & 360 (7th Gen) took hold in 2006ish
and so on
PS3 & 360 won't be "retro" until PS5 and XB2 come around... maybe up to 10 years from now.
with all that being said, the goalposts may alter (subconsciously) as the gaps in console tech and generations becomes less obvious.
The gap between PS2 and PS3 was smaller vs PS2 and PS1. More obviously, the gap between XB1 and 360 is smaller still.
Presumably, things will plateau at some point anyway...