How to make an account active or inactive?

I am looking at possible ways to define an account as 'active' or 'inactive'. A simple checkbox will do the trick to identify this however I am stuck on how to define it as a process (looking to automate). Initially I thought about defining it as: 


Accounts>Date Modified - 3 weeks 

However that poses an issue because an account does not necessarily get updated when a related record is added/modified.

Has anyone done this before and/or have some insight how I could go about it?