Fitness age.

It is not specific to the garmin Users.  There have been a number of studies used in the past (likely first big one was in Norway)to determine something called fitness age.  You can find calculators online,  some based on nothing more than a series of health/activity questions, others based on more specific performance factors. Most of Garmin' performance metrics are based on FirstBeat algoriths so I would expect this to be the case here.  FirstBeat https://www.firstbeat.com/en/consumer-feature/fitness-age/ - you might be able to dig deeper as they do sometimes have white papers - not sure if their algorithm is also using weight or other factors other than VO2Max. The fact that there are multiple ways and systems to get your "fitness age' should tell you that it is not really providing any focused performance number other than bragging rights I suppose.  Also given that it is based (with FirstBeat) on an algorithm/model to estimate your VO2Max also tells you something.  I say all this but I do brag about mine (nobody listens but...)