In the US it's a really common expression in informal usage, but it's usually not used for talking about people in the way you used it. Getting food delivered didn't use to be a thing. Self-checkout lanes didn't used to be a thing. When I was a kid, 24 hours stores weren't a thing. Just remember it's a very informal expression.

Grammar: Don’t give me a lecture Don’t lecture me It also doesn’t make a lot of sense from the point of view of its meaning. Being born isn’t sufficient to know what anime was popular in the 90s. Maybe you mean something like: You’re too young to know anything about 90s anime. You weren’t even alive in the 90s. How can you know what was popular then? I loved anime in the 90s and can tell you what was popular then. Jonathan’s examples of how to use ‘a thing’ are perfect. It doesn’t have a place here.

The sentence doesn’t have a precise meaning. I interpret it as “you hadn’t become noticed yet”. That could mean you hadn’t been born. It could mean your personality had not yet burst forth. It could mean you hadn’t become famous yet. There are many possibilities. The dictionary meaning of “thing” doesn’t apply.