I'm assuming you mean the US spends 33% as much on those under 18 as it does on those over 65 (otherwise your comment about voting makes no sense)? If so, it seem sensible for the government to spend more on the elderly, simply because they cost most and there is nobody else to pay for it. People under 18 typically have one or two parents to care for them. Older people don't have parents and don't necessarily have family to pay for their housing, food, etc. Then you also have to factor in that healthcare costs for the elderly are necessarily more expensive.

Not saying kids don't deserve to be well-cared for. I was a welfare kid and I'm glad there were welfare programs. Just saying I think we'll all be glad we can also get some help when we're over 65, especially if we can't count on our family (I have no siblings and may not have children).