Most young people are idealistic, myself included, and if we're political at all we tend to be swayed by the idealism of the left as opposed to the hard work of the right. The left seems to believe that all things are possible for all people and that the government's primary function is to supply all things to all people. So, naturally, lots of young people see government run by leftists as their ticket to a good life. Over time, however, most young people become realists once they see that not all people can get all things and that the really good things, materially, come to those who work extra hard to get them. And along the way, those who "get it" understand that those on the right are trying to get the government to concentrate on the basics and get out of your way while those on the left tend to envy individual success and use government to take some of that success away from you and give it to those who aren't as successful. And if you work hard in your own life, trying to achieve success, you begin to distrust and down right hate those who are trying to limit your success and/or take some of it away from you.
In effect, as you progress from young idealist to older realist, you either succeed as an independent individual and become more likely to lean to the right or you have less success as an individual and become more dependent on the "collective" to grant you success and more likely to lean to the left.
There are two small exceptions to this fairly simplistic rule - those who feel guilt associated with their enormous individual success and lean back to their idealistic left late in life and those who have little success but dream it can still happen and idealistically lean right because they still hope to succeed and don't want the government to take it from them.