It's going to vary for everyone, but retro basically just means old stuff, so it's always a moving target.

When I was playing the NES, games like Pong and Donkey Kong and Pac Man felt retro. When I was playing the N64, NES games felt retro. Nowadays, PS1 and N64 definitely feel retro to me. I don't think I'm ready to call the Wii retro yet, but I know most people probably accept it as retro now.