Different models will naturally have different firmware, including different versions. In general, keeping all models on firmware of the same era is advisable... typically this would be the latest greatest.
That said, it is always prudent to review release notes to know what's up and for how long, and if you have the time to save time and trouble, check around to see how the latest greatest is working for other users to avoid any new surprises/defects.
With practice, you can observe similar release note entries and assume similar stages of firmware development... might help.
Yes, it matters. When the firmware is that old, it does.
You owe it to your network security to at least attempt upgrading the firmware to current levels.
And, if there are issues, you should also be ready to perform an M&M configuration too after a full reset to factory defaults too (this is the only way to properly test new versions, or at least, to be able to troubleshoot them properly).