Last 2 guys, it's a simple mathematical problem. In Socialist Utopias the state takes from those who work and gives to those who do not. The baby boomers are retiring. They have not sired enough kids to take over the bills. This means less funding for more people, which means higher taxes and less payouts for each individual. Everyone loses. Matters of environment, ethics, lolcats and whatever else is entirely irrelevant. It's a problem that will bankrupt all of Europe even if there were no other problems, and it will take generations to fix if we started right now.
This isn't entirely true, the production rate is higher than before, even with less people.
So if you need less people to produce more stuff, it could be possible to solve this.
It's actually not. Less than half the population in my country works. Everyone else is on some form of public handouts. More importantly, we are dying. With each generation being smaller than the last and immigration ever increasing, it's just a matter of time until some of the european countries lose their native population entirely. The culture is already shot in many places.
Since you are obviously not European,
I'm danish.
Never assume on the internet.
The US problem is even simpler and has been covered.