My definition of a vet would be somebody who knows what they're talking about... Not character skill. Sure, you could have a totally GMed tank mage, have been around for 3 years and own 3 towers... But like somebody back there said (I think it was Duo), if you can't even do something like walk from Yew to Cove, or know all of the small interesting bits that make the game worthwhile, then you're not really that good, are you?
Note: If that sounds like anyone you know, disregard it, it was only an example
Personally, I would class people like Lister as vets. They know a lot of stuff, they're willing to share it, and they help a lot of people whenever they can.
Also Duo... I know a lot of people don't see eye to eye with him, but he's knowledgable, and he helps.
So I don't think it depends on how long you've been around, etc, although some background experience on a shard is always good... It always depends on the individual person.