It means it's bad for everyone, even worse for teenagers

In this example you could replace "never mind" with "not only" or "not just". It means it is not only bad just for teenagers but rather for everyone/anyone.

I a agree that it clearly implies that. I wanted the person posing the question to understand that it is only in context that it means that by implication.

So "never mind (something"), means forget about it , stop thinking about it, cast it from your mind. Other examples. "Oh, never mind that, you sound be more worried about..." Mind being used to mean think about is used in another idiomatic way, usually in a passive-agressive way if someone does something without thinking how it will affect you: "Don't mind Me!"

So if someone said isolation from lockdown is bad for teenagers, you could say: Never mind teenagers, it's bad for anyone. Which implies it is the same for everyone, or: It's bad for anyone, never mind teenagers, which could imply that it is worse for teenagers.