From what i know, a "live" asset is one that responded during the last scan ran against it. When you age out a device is really up to your environment. I'd doubt 2 years missing from scans would make the data on that system be considered valid.
Live assets actually refers to MVM's Active Asset list. This is the list of assets MVM considered active based on your Asset Identification rules. At the bottom of your Asset identification rules there is an "Asset Activity" rule, which lets you define "Number of days without being discovered before an asset is marked "inactive"". Any asset that hasn't been seen in the number of days listed in this rule is marked as inactive and would not show up in your "live asset" list.