This is due to the call at line 44 to EntityItem::getAge which is actually a client-side only annotated function, despite the fact that the value is updated on the server side (which always seemed strange to me).

Unfortunately, I think your only option at this stage would be to use an access transformer (such as public net.minecraft.entity.item.EntityItem field_70292_b # age) to make the actual age field of the EntityItem public, allowing you to query that instead.

Thank you very much for the report and the pull. I'll merge it asap.

This is now uploaded on curseforge (1.12.2-1.1.1.9).
Thank you very much for the help.
~Johannes

No worries! If I notice anything else while playing around with the mod (or while people on my server are), I'll be sure to report it and submit a PR if it's something I can easily fix.