Bricu of WTT: [RP] put forth a pretty thought-provoking question the other day: How old are your characters? From one point of view, that's pretty simple. You do a quick /played, report back the number of days it returns, and there you go. But, especially for those of us who roleplay, it can sometimes go a bit deeper. Sometimes, we take our characters across games.
Of course, this doesn't mean importing characters completely wholesale, you understand. On the RP end, you're still going to want to convert them to be born and bred citizens of Azeroth (or Outland), and either way, you may not be able to convert them to the exact race or class they were. There's just no Human Rangers or Halfling Monks in WoW, more's the pity.