Bridget,
You could create a smart list to track this. Here is an example. You will just want to change the filter to 18 months instead of the 180 days I have put there.
This could take a long time to run so you might want to create a smart campaign to add the leads that match the criteria to a static list.
Let me know if you need more directions.
-Thomas
![0EM50000000TCIl.png 0EM50000000TCIl.png](/t5/image/serverpage/image-id/12817i3EE2D1573DED2A3C/image-size/large?v=v2&px=999)
Here's an example of a smart campaign flow step to add to a list: