Old people all have massive long term health issues. That's where most of the healthcare costs in universal healthcares go. Change the balance way more towards old instead of young being generally still healthy and feeding the system financially, and it just doesn't work anymore and becomes unsustainable.

There are partial solutions, ie adding some small fees for each visit (since a lot of retirees are lonely and go for the doctor visit just to talk to somebody or complain, and as I said they all have various mix of long term issues). Where we live healthcare isn't fully free and you have to chip in a bit, and this ain't such an issue (apart from wealthy old people).