[tptacek]

You're not making sense. If heritability means genetic determination, as you say it does, and genes are fixed at birth, then heritability can't change as you age.

None of what you're being told is first-principles axiomatic reasoning. This is all stuff you can just go look up. You got so close with that Wikipedia definition of heritability! All you need to do now is understand what those words mean.