Is it correct to ask, 'How old were you being when you first visited a Chinese restaurant'
Or being is not necessary here?
Using of being here was incorrect that it wasn't necessary.
Have I got this right?
Ok. Would you omit it?
I am sorry I haven't read thoroughly the reply before yours ...
How old are you?
How old were you?
How old were you when ... ?
How old will you be when ... ?
That's it. We do not use another verb after "are you/were you/will you be" when talking about age. It's not a case of omitting it by choice.
You could devise a context where it would be possible to use it, but it would be highly artifical- an actor playing a character at a certain age might just about work. In an ordinary question, it's an error IMO.