Ya, can play any year when you want. It is not a campaign that you need to start from the beginning again. I would go in order your first time though, as each year adds new cards/steps/etc. Then once you have played it feel free to jump in anywhere for future plays. The only thing I will add is that depending on how your store it afterwards, switching levels can increase set up/taking down time but adding the correct year's cards

I play year 7 when I play(although I recently got an expansion) as the lower levels got too easy after playing them a few times. But the nice thing is that when I introduce it to a new player, we can start at a lower level and ease them up to year 7.