You're right, policy decisions don't cause more people to have kids at a young age.
'Abstinence only' sex education causes more people to have kids at a young age, because it prevents more people from learning what options there are available. Add into that the issue that young people have always been difficult to stop fucking each other throughout history, and combine with the highly sexualised modern society, and 'Abstinence only' sex education is, frankly, evil.
You're right, policy decisions don't cause more people to have kids at a young age. They do however, have an effect on joblessness [1], just to name one facet of the economic environment that now exists in the US.
[1] http://www.google.com/publicdata/explore?ds=z1ebjpgk2654c1_&...