My state is 14.
This is not actually at all true, but I'm not really going to share my
total number of living relatives that are younger than me.
Even though condition 2 is subject to argument (this state could theoretically go down (or even up) by two or more at the exact same moment), I think this must be the intended state, because there are very few states that are guaranteed to satisfy condition 4 in a manner that makes the "As long as you live" modifier meaningful.
Here are all the clues:
Each person has a single state at any point in time.
Allowing for the possible exception of Dr. Schrödinger, the state is well-defined for any person at a given time. (We don't currently have enough people travelling at relativistic speeds for that to become an issue.)
They can move one state up or one state down at any point in time.
When a relative is born, the state goes up by one. A death may bring it down by one.
The wording excludes the answer "number of living descendants" (which was my first idea), because that number cannot possibly go up before puberty.
They can change states after they die.
As long as you live, your state is lower than your mother's.
As long as you live, your mother's state is at least equal to your state plus one (you). After you die, it's possible for the states to become equal, which they will be after all relatives born before you but after your mother have died.
I don't know what the highest state currently held is, but I can put an upper bound on it.
Definitely not more than the current human population.
Some people claim that they know the highest state, and who holds it.
Some people would probably claim to know the earth's population and claim they are all descendants of Adam and Eve. Since I don't like the implications this has on the mother of Cain's children, I'm going to go explain this one with Genghis Khan instead. (Here's an article where someone is trying to estimate the state)
Most people know their own state.
This fits the "direct descendants" answer better, but it's definitely not rare to know how many younger relatives you have.
Chuck Norris was born with state 0.
Everyone is the youngest one when they are born