So what truely defines a newbie so that when the term gets used here we have an idea of whether this person has just started or has been doing it for a number of years
I don't think there is just one answer to your question - if I have understood it correctly.
Newbie is used to refer to someone new to the forum - and/or as someone new to the hobby. Usually I think people mean the latter. Further, there isn't one, single, definitive point in terms of time or experience, I don't think, at which you switch from "newbie" to "experienced"
A newbie *here* in terms of the title under an avatar is simply an indication of how frequently the person has posted on the forum (less than 30 or 50 posts, I think - can't remember which) and that may or may not be commensurate with their knowledge of fishkeeping. So being a newbie to the forum doesn't mean that one is a newbie to fishkeeping. Just as a person who has posted enough times to be a "leader of the fishes" may or may not be talking out of somewhere much further south than their mouth every time they post. One thousand posts in three weeks all of them absolute rubbish - or even sensible questions - does not of itself negate one's newbie status other than in the sense that it changes your board title
There are many, many members here who, like yourself, come to the forum as anything but newbies regardless of what their title might say. That generally becomes pretty clear pretty quickly through the nature and quality of their posts - and if they are active on the forum, the newbie label under the avie will be gone soon enough anyway.
Finally, of course, although generally everybody is learning all the time, and none of us knows everything, it is possible to have years of experience in something ( not just fishkeeping) and yet to have learned next to *nothing* from them.....I don't know if I'd exactly call them a newbie though.