Achievable Fitness Age

Yes. This function has chanegd again though I can see the page if I refresh the browser. My issues is that they have changed the advice as it now says just 5 mins vigourous activity in a day will count, however it will not still not count days with less than 30 minutes. I lookes up the support pages in Garmin Technology, and the advice there is that 15 vigorous minutes are required to count and they recomemnd 5 vigourous days a week which makes sense as 75 vigorous minutes a week are also needed to improve fitnerss age. All in all, it is a bit of a mess.

Hello 7494571.  For the 404 error that you're seeing, I encourage you to clear cookies/cache/browser history on your web browser and/or try a different web browser all together.  I was unable to reproduce this 404 error.

As for the differences that you're seeing in Fitness Age, please see the "My Device Supports Improved Fitness Age, but It Is Not Displaying One or Updating" and "Why Did My Fitness Age Suddenly Change?" sections of the FAQ below to see some reasons why the value may have adjusted or is not updating:

What Does Fitness Age Mean in Garmin Connect?

Please note that, depending on what model watch that you use, Fitness Age may be calculated differently.  See the "Improved Fitness Age Using Additional Factors" section if your device supports improved Fitness Age.

My bad - I misspoke and jumped to the '404' conclusion.  In actual fact, URL connect.garmin.com/.../fitness-age continues to display a page containing "Page Not Found" and "We're sorry. The page you're looking for no longer exists." neither of which necessarily mean an HTTP protocol break.  I didn't try to debug the web page to see what kind of query it is actually trying to make.  FYI - I have a Venu 2, and I'm sorry to hear the fitness age calculation varies across watch models.

My watch supports improved fitness age using additional factors, and it does display my fitness age. My issue is that Garmin set clear criteria for vigorous activity days and monitors progress, but despite meeting the criteria, it will not include days unless they have over 30 minutes of vigorous activity when the guidance says that days with 5 minutes or 15 minutes will be included depending on which Garmin advice you read. 15 minutes makes sense so I have been adhering to this. It is definitely a bug. 

Update - purging browser site cache/data/cookies per @Garmin-Kevin recommendation made several "Health Stats" pages, including the fitness-age one I specifically identified, work again.
On a related note, my fitness age was higher than expected in the past several days, in spite of changing my "Activity Level" to 10 (but leaving my chronological age alone) and it turned out this was due to use of BMI instead of Body Fat % to compute fitness age. Upon inspection, my most recent Index S2 weigh-in had no recorded BF%, and as soon as I deleted that bogus weigh-in, my fitness age jumped downwards by 3.5 years.  The Index S2 is being bashed adequately in a separate thread.

Hello MoFergus.  Could you please gather some screenshots of what you're seeing for the advice and criteria that you're mentioning?

Also, so that I may assist you further, please reply with the following:

1. May we, if necessary, have permission to email you?
2. May we, if necessary, have permission to view/access your Garmin Connect account?
3. In what country do you live?

Thank you MoFergus.  Can you please share the link for the FAQ (the 2nd screenshot)?

Thank you for looking in to this. Here is the link. 
https://www.garmin.com/en-GB/garmin-technology/health-science/fitness-age/

Hi, Garmin-KevinGarmin-Kevin - any updates on this topic, please? Thank you!