By the time the millennials are old enough for their votes to matter, they are going to be close to collecting social security. Its not going anywhere.
Not if you actually figure out the tipping point, which (based upon the size of both generations, and the rate of 'early' (before life expectancy) death of Boomers) almost certainly occurs before 2020, the year the last of the Millennials actually can vote. And the tipping point is actually likely to occur sooner, maybe even next year, due to the fact that the elderly typically stop voting (on average, which is the only context that matters here) before they actually die. This is why, I think, the Gen Xer Republican candidates for president (such as Paul and Cruz) are pandering to their generation.
Besides, my experience is that Millennials far are more liberal than their predecessors.
Only if you are stuck on the left-right or liberal-conservative dichotomy. Millennials, like pretty much everything else, are more complex than that. In general, Millennials are socially liberal but fiscally conservative. The socially liberal part has, thus far, dominated the public debate; but the Repubs are trying to turn the topic towards fiscal issues. This is exactly what Rand Paul was trying to do with his 14.5% 'Fair & Flat Tax' plan. Only time will tell if it will work.
By the way, the combination of socially liberal and fiscally conservative is one definition of a libertarian, for which thought Rand Paul is the reigning representative in our political climate. I can't even think of a Democrat that could claim to be a libertarian anymore. The book, The Fourth Turning, predicted most of this; right down to the political bent of Millennials, which scared the crap out of the authors, since they were (and still are as far as I know) quite liberal. It was published in 1997, and the follow-up book, The Rise of the Millennials, predicted their early dominance of the electorate. The polticos know this already, and are planning accordingly.