Could it possibly be that the costs of fixing all health problems of old people simply skyrocket after some age? Thus, given limited resources, we only have the following options:

• cure as much as we can, and watch the whole national budget spent on healthcare for old people;
• decide that some things will not be cured, at least for average people; or
• have mandatory euthanasia at some age.

I am not saying that even if this is the case, that the decision where to draw the line was optimal. Just that a line has to exist somewhere.

(And it's not just old people, of course. There is always a surgery that could save someone's life, but won't be done for budget reasons. Etc.)