Ah, understood. Doing this dynamically is a little difficult. Moving forward you can add every Lead to a daily recurring batch campaign that in one year you can change a custom Mkto field, something like "Lead > 1 yr", to true. Then the list will be up to date for any new Leads.
For the past, I haven't come across a clean way to do this. Not saying there isn't one, but I just haven't seen one.
You could create a campaign that changes the value of the custom field I mentioned above on a weekly basis. Add any Lead created within the past year as the smart list, then in the flow steps, any Lead created before 3/12/13 change the value to TRUE, wait 1 week, then any Lead created before 3/19/13 change the value to TRUE. This will take a little time to set up, but once you are done, you won't have to worry about it again if you combine this with the process Imentioned above moving forward.
Pretty clunky, but one way to deal with it.