Morning, everyone. This is an interesting idea and, as always, we're keen to hear what people think.
It's not something we do at the moment, partly because there are a few flaws in the plan - one being that we would worry that any posters who were marked out as new might not be taken seriously or, worse, it would be assumed they were trolls. The fact that no one knows how long someone has been on the site does at least put everyone on an equal footing. We've also never gone in for the ticker tapes or 'badges' that mark people out as being 'veterans' of the site for the same reason.
If you're ever concerned that a new name you've spotted might not be genuine, you can always hit the report button and it's fairly easy for us to see if they are new or not but, more importantly, whether there are other factors that lead us to believe they aren't genuine. There are new users here every day who are perfectly above board, so there's definitely a lot more to it than 'length of service' but where we can see other flags as well, we can quickly sort it out usually.
As we say, there are some interesting ideas here though, so please do keep them coming and we'll keep reading. We do already have some limits on what 'newbies' can do, such as posting in the sex topic, for example, so there may well be other tweaks we can make like that, which would have a positive effect.