The "Latest Comment" field on the Aged Invoice by Contact was not displaying any information. To populate that field on the Aged Invoice by Contract report, do the following:
1. Go to the Administration menu - Codes and Types - Comment Type
2. At the bottom of the screen you'll see a box labeled "Include latest comment on Aged Invoice by Contract."
3. In order to get these comments to show up you'll just need to check that box for the particular type of comment that you want to show up.
4. Note that it's only going to show the most recent comment. For example, if you're looking for Collection comments but the most recent comment is a Bankruptcy comment type it's going to show the Bankruptcy and not Collections.