So a state can infringe on a person's liberty, but not life, based on his age? Patrick Henry said the former was more important.
Your question is valid, though. I'll attempt to answer. A person is a single person when he can live independently. Prior to that he is two people, the oldest of whom has a IVth amdendment right to be secure in her own individual person. I (and I should also say the 10th Amendment) leave it up to the State legislatures to decide when that point is, constrained by the US Constitution, their State Constitutions and State Courts. States can decide when liberty and life begins. The Constitution has prohibited the States from considering race, prior condition of servitude, and in the case of voting, sex, ability to pay poll tax, or being 18 or older, when deciding. But the states did not delegate that decision making power to the Federal government (unless you consider the 14th Amendment valid :).
Even, hypothetically, if the 14th Amendment is considered valid, there is another amendment, unique in that it restrains individuals (not government) from acting on individuals, and that is the 13th Amendment. I hadn't thought of it before, but there is a right in there, of a mother to not allow her body to involuntarily serve the needs of another's. Babies can't harness their mother's bodies for labor. 14th Amendment due process of law requires the mother's liberty to be protected as well, so the 14th Amendment cancels itself out.
Without life there can be no liberty, Patrick Henry understood this.
A person is a single person when he can live independently.
This proposition is entirely incorrect because this would give the states cause to kill children until the age of, lets say about, 8.
Prior to that he is two people,
This statement is also incorrect and the human genome proves it indisputably. The argument that a baby is the same as an appendix is fallacious.
As for your 14th amengment argument, I don't agree with it. States need to be constrained just like the federal government does. If man was incapable of evil, then government would be superfluous but its not. In general, I support federalism and the tenth but not where generally agreed to unalienable rights are concerned.
The right to life should be protected by the constitution of the United States and I think it is. The right to smoke dope should be left to the states.