The younger generations are interested in politics, they perhaps just don't realise it (they may be concerned about how they will ever afford their first house, or the level of debt they might incur going to university, or many many more issues - but don't necessarily link that to a political debate). So perhaps the messaging / targeting is wrong. I'm a strong believer in mandatory voting (but in normal elections, rather than referendum, I also believe there should be abstention box you can tick).
In terms of immigration, the internal movement of labour within the EU is going to cause the more prosperous countries bigger and bigger problems over time, particularly when Turkey gain entry. What has membership of the EU got to do with "non-EU" immigration? Well, Belgium (if memory serves), allows you to apply for citizenship after three years. So after living in Belgium for just three years, you can apply for citizenship and get a shiny new Belgian passport. Which just happens to grant you free movement throughout the EU. Some of the newer Eastern European countries don't have the same control over passport stocks as the UK does either - so regardless of where you are from originally, you can buy what is essentially a fully valid passport - which again grants you freedom of movement throughout the EU.
I personally welcome immigration - regardless of where someone might be from - but do believe we should control the numbers so that a) we can maintain net immigration at a sustainable level and b) we prioritise people with the skills that we require. Who on here believes we should just be able to go and live/work in Australia? I'm sure we're all accepting of the fact that we have to score sufficient points to get the right visa. So why wouldn't you support the same process for your own country?
On multiculturalism - an integrated approach I have no concern with, but this has been a poisonous and failed experiment from the Blair era, and you often see isolated / ghettoed communities. You should take a look at the birth statistics as well of some of these immigrant groups. If you care about the sustainability of the country, the NHS (and the cost of running it), education and lots of other local services, you should be concerned. I have a good friend that is the ex-Chief Registrar of England, and some of the stats are eye-popping.
ps/ I also believe that there should be an English Assembly/Parliament, for those issues that only relate to England.