[0xy]

If you wanted to protect hospitals, then why were 15 year olds locked down and not primarily old and obese people (the vast, vast majority of the risk).

15 years old took the brunt of the lockdowns yet had almost zero risk from the virus.

Why is that? Shouldn't the people at actual risk of the virus shelter themselves?

The difference in risk between a healthy 15 year old and an obese 85 year old is orders of magnitude different. The virus might as well be the common cold to the former.