This is what Garmin's support website tells about the Fitness Age feature:
To determine whether Fitness Age is a compatible device feature that will display this info in Garmin Connect, refer to your owner's manual or the specs page for your device on Garmin.com.
So if it does not show up in Garmin Connect, then the feature is likely not supported by your device. To be sure, check out its specifications, and/or the user manual.
Update: Removed the Fenix 3 HR, coincidentally there's an GC update in Play Store, installed that and still no Fitness Age.
Yes the Enduro is. Checked this during setup and double checked. Maybe I should remove the F3 to see if this makes a difference.
Update: Removed the Fenix 3 HR, coincidentally there's an GC update in Play Store, installed that and still no Fitness Age.
In that case I'd contact the Customer Support and ask them to look at it. Enduro is a new model, so they perhaps just forgot to include the option in GC for this new model, and nobody reported it yet.