The thing about the elderly, though, is that once they die off there tends to me another generation of people who've become elderly in the interim.
The way I see it is that laws shouldn't specifically be framed around a particular age group (or any other division in society for that matter). That may on occasion mean that the "right thing to do" ends up aggravating a certain kind of person. That's democracy, though -- someone always ends up losing, as it's not possible to satisfy everybody.
You're wrong -- in principle, at least, if not in practice -- about the "[having] to change laws, make special provisions for [immigrants]". In the first place there's no special provision allowing Muslims to build mosques, as this is just covered by freedom of religious expression (and then planning laws afterwards). As long as freedom of religious expression doesn't contravene other, more universal laws, then this should be completely uncontroversial. In practice, we probably have tipped too far in some ways into allowing "freedom of religious expression" in some cases and not in others, although inevitably change takes a while to settle and in the long term I expect things will balance out again -- as indeed, is already the case for the majority of migrants.
Lack of integration and interaction with migrants is also an issue, and I don't think that anyone on the "left" would argue with this either. The difference again is how to approach this. Those on the right seem to believe that the answer is to cut off the inflow (almost) entirely, but seem silent on what to do to improve the situation with people already here. I'm also going to be at this point, because the best I can come up with is some kind of hoary old cliche -- but, at any rate, taking an instant distrust of immigrants just because they are from another country is counterproductive.
The current immigration levels we are seeing are unsustainable to be sure, and I am all for active border controls so that we are aware as far as possible who is coming in to the country and what they are doing once they get here. I don't see it as an invasion, though. Invasions imply aggression and hostility, and (at least from most immigrants) these two things are lacking.