Good question! If someone is using the fact they joined in 08 to convince you to trust them it doesn't sound to good. If someone is trusted you shouldn't need convincing with such trivial things. You could've joined yesterday and be the most honest person in the world or you could be someone with no morals who hasn't had a reason to steal for 5 years.. see where I'm coming from?
If you're talking about an account then unless the user is HUGELY reputable and is the original owner then don't bother IMO.
Otherwise.. look at the vouches; the number of vouches, amount traded and order of trade are the aspects to look at, from least important to most important. Number of vouches is the first thing to look at.. if there are only a few then the person is a new trader and a safe rule of thumb is to use a middleman for the trade. If the user has a fair amount to large amount of vouches then you should look through these for the trade amounts; bearing in mind that 50 vouches at 10$ each are lower quality than 10 vouches for over 100$ each. Lastly, the order of trade is most significant. It's not uncommon for people to look trusted by garnering vouches just by buying something and going first; meaning the other party isn't putting any trust in them. So if they've been going second (meaning the other trader trusted them before payment), they're of a substantial amount and there are a lot of them: then I'd be confident in trusting said person.
Hope that makes sense.