You really don’t get it do you?
You won’t get infected by an old person. You will get infected by someone you work with or someone you live with. Old people are not a threat to you. If an older person gets infected it will most probably be from a youngster.
Getting this virus is not a death sentence, far from it.
There are nowhere near enough ICU beds for the numbers that will need them, something like 3,700 out of tens of thousands. It won’t make any difference if those over 70 self-isolate. Most of them never go anywhere near a youngster anyway.