I tried setup Cardio fitness notifications on my iPhone XS Max and my Apple Watch Series 5. It says required 20 years old or older. Like my Birthday 2024 for ECG, I have to waiting for my Birthday 2022 to get Cardio fitness notifications.
Not sure why you created a thread stating the obvious. Per your screen shot, Apple tells you right away you have to be at least 20 years old. If you want to complain, contact Apple.