What's a system you didn't know until it was retro? I have an Intellivision and an Atari 800XL, never used them as a kid, now they're mysterious and weird to me.
I grew up with the Atari 800 series, but even though I knew about it as a kid, I didn't get an Intellivision for the first time until I was in my 30s. It's now one of my core vintage systems; I've always got one hooked up.
Atari 8 bits. I never had one growing up, and I only had one friend with a 400, but by the time we were friends it was the NES era. Everyone else I knew had Commodores or Apples.