Certainly time in the game contributes to being a 'vet', but I think I'd take it further.
I think 'vets' recognize the game for what it is, a game. I don't think the 'vets' get caught up in the stuff that gets folks early in the game, as in score mongering, flying only the uber rides and thinking that being the high scorer and gaining rank means much if anything.
I'd like to believe that 'vets' try to contribute positively to the AH community in whatever ways they can. This can include how they carry themselves in the arena and on 200. It can be in helping newbies learn the ropes. It can be on the boards with advice or help. I can be through their work doing aircraft skins or maps. It can be through all the special events stuff, scenarios, snapshots etc.
Basically I think you've made 'vet' status when you recognize a responsibilty back to the game and community and do your best to make it better.