What are Oldtimers and Youngtimers in this context? I'm not in the vintage car community, so I'm a little unclear.
You should be free to create whatever community you want in my opinion. If you build it, they will come.
If you're concerned about vitriol over cars because of, say, the Fuck Cars community, I think the unfortunate reality is you're going to see some of that no matter where you go. Don't let internet strangers' opinions keep you from liking a cool thing. I'm not very into cars myself, but I do like old vintage stuff, and one of the things I love about Lemmy is seeing vintage everything.