Recently I got the following question from my customer:
Can I mount a datastore formatted under vSphere 6.0 to the vSphere 5.1 or 5.5 host?
I answered: yes, it should work fine because there is the same major version of VMFS but I needed to confirm it, it's me 
I have not able to find any information about it so I decided to check it. vSphere 6.0 introduced VMFS version 5.61 and for a quick reminder we can meet the following VMFS 5 versions:
vSphere 5.5 - 5.60
vSphere 5.1 - 5.58
vSphere 5.0 - 5.54
I could not find any changelog about difference between above versions 
I did a quick test and created a small datastore under vSphere 6.0 and later mounted it to vSphere 5.5 U2. It worked without any problems:
To check VMFS version from the ESXi CLI, please use vmkfstools --queryfs -h /vmfs/volumes/datastore_name Of course, you can also check via GUI (Web Client or vSphere Client), however I prefer CLI 😛
You can not use any tool to upgrade/convert VMFS 5.54-5.60 to 5.61. If you use vmkfstools -T you will get the following error:
/vmfs/volumes/xxxxxx with version 5 cannot be upgraded.
Error: Operations not supported
To upgrade VMFS 5.60 to 5.61 you need reformat datastore manually, but for what?
If you still use VMFS 3, you can use the above command to upgrade the datastore to VMFS 5.