If you had to pick an age for societal burden, it'd be the average "young" person who isn't making a net-positive contribution until around the age of 25. Most people don't receive a corresponding 25 years of retirement, and the retirement they do receive is a product of contributing to it for 40 years.
But it turns out young people become middle-age people, become old people, and we're all in this together. The real problem is not about how the pie is split across generations, but about the realities of lifespans, economic production, and expenses. If you're responsible for funding the entirety of your retirement, all of this is abundantly clear. When you nationalize retirement, all sorts of budgeting tricks start happening, like "borrowing" to fund other programs, papering shortfalls via population growth, etc. Then you start getting age warfare when the govt has to eventually cut back.
You could dissolve the national retirement plan, but that seems like a bad idea for similar reasons as entirely dissolving national welfare and insurance programs. It will always be the case that some people, need some help, some of the time. I guess in my ideal world, it's reduced to a much smaller safety net, because the government is managing the economy well enough that the populace has the wherewithal to save appropriately, and they are educated enough to do so.