How do I determine the age of a virtual machine?

I've seen some Powershell to go through all the events and find the creation event, but this method doesn't seem ideal, as I understand it can get confused by software version upgrades, and the events may not be retained depending on settings.  

I can get the creation time of the first root snapshot, e.g. vm.snapshot.rootSnapshotList[0].createTime.   This is quite useful, and coincides with creation dates for many machines, however some VMs don't have a root snapshot.

Can I do better?

Thanks!

PS:  I am using pyVmomi, however I don't think this is the problem, and can switch to another language if necessary.