I'm 54 and have had my Garmin (Fenix 5x) for 2 weeks.
Week one was pretty exhausting with some hard hikes as well as my 3 runs Garmin tells me I have a fitness age of 64
Week 2 just the regular 3 runs and a very hard hike yesterday with a lot of exertion because of the gradient. Apparently I now have a fitness age of 71
Another question, My Garmin tells me I'm between 93% and 96% of my Max heartrate when I'm running, does this seem high and does it adjust itself over time?