change goal fitness age

You can't change it by yourself. It is "an estimate how fit you are compared to your actual age." See attached support document.

Fitness Age | Health Science | Garmin Technology | Garmin

I was wondering the same, my minimum fitness age is 41,5, but I feel I could get even less

You can't change the target/goal/minimum fitness age, it's based on Garmin's determination (definition?) of your absolutely lowest realistic fitness age. (Whatever "realistic" means for a completely made-up metric, anyway.) (To be fair, the word they actually use is "achievable", not "realistic".)

The history behind this is there used to be no minimum fitness age (or the minimum was around 20), which led to endless posts from 40-70 year olds humble-bragging about having a fitness age of 20. While this kind of result might've seemed impressive on paper, it also probably called into question the usefulness of the fitness age metric in the first place. So it seems that Garmin introduced a minimum to make the feature more realistic.

I reckon the minimum fitness age is currently no more than 11 years younger than your actual physical.age. That's what I'm telling myself anyway

1153848 said:

I was just wondering why my friend had a fitness age target 11 years younger that he actually is, and for me is not possible more than 5 years younger, that is.

Are the 2 of you the same age?

I can think of two possible explanations:

- if the two of you aren't the same age, then the fitness age target could be based on your specific age. (i.e. perhaps everyone who is the same age has the same target). Maybe, to further your point about 70 year olds, the older you are, the bigger the difference between your actual age and target age?

- if the two of you are the same age, maybe there's a few years of "wiggle room" based on factors we don't know about

1153848 said:

I’ve seen people around 70 y.o training that would have deserved to achieve a 40y.o target

Garmin would beg to differ ¯\_(ツ)_/¯