Another way to increase younger population is also to encourage immigration in that population group. However, for other reasons, this will never be acknowledged as a reasonable measure :)
There isn't much anti-immigration sentiment in the US. What we have in the US is anti illegal immigration sentiment.
In terms of European countries there was an article about a week ago regarding Americans going to Germany for free college. The Germans were hoping they'd stay.
I think people worry about the quality of immigrants. We want more young Sergei Brins and fewer middle aged guys with a sixth grade education.
While that may be technically true, AFAIK it's hard for someone to immigrate to the US. If I wanted to move to the US tomorrow, I believe it would involve me having to get a job before I got there.
One could argue that saying that you're against illegal immigration and making legal immigration very difficult is the same as saying that you don't really want immigration.