dhcp-server lease age

Hi
Can anyone share the rules around the implementation of the new field “age” in dhcp-server lease details in V7.
I see it only populates the 2nd time a device associates with the server and then only has the current lease “uptime”.
If the lease is made static, will it count from the initial association ?

I cannot seem to find any documentation regarding the field, just going on what appears to be obvious.
Rgds