I really like the idea, but I don't think it should be anything to complicated like activity etc. It would just be a neat way of looking and someone and thinking "hey, that guy's been around for quite some time" and nothing else.
I, for example, haven't played for at least 4 months, but I've always been reading the forums and I will certainly also have some more active periods, I'm just a little busy playing payday2 right now
So basically, just make a tag for people that have joined a long time ago (probably using the forums, I don't think we have acutal game data for those that started playing when the member system was not even in place, do we?
)