This is a great question with no definitive answer 🙂 There are a number of ways of looking at it, though the ultimate answer is whatever each person considers retro. A few general ways to look at it include:
whatever was around when you were a kid. Of course, this depends wildly on who you are asking - for me this would include the Tandy Color Computer / Commodore 64 era through 486 PCs (or so.) For other people this could be a little older or much newer of course.
Some fixed definition like (anything before the year 2000 or no PCs etc.) I’m not a huge fan of this personally as it leaves some people out.
Any computer old enough that a single person could fully understand it’s workings and diagnose/repair problems etc. This generally includes 8 and some 16 bit systems. I’ve increasingly come to prefer this view because beyond nostalgia it also helps define something that I believe is a significant benefit to these systems.
Normally I would say anything over 10 years old. But due to how 10 year old computers are fully useable, I am thinking anything with 1 core. Maybe in a few years it could be ones without a TMP2 :)