Potential really has little to do with it, and did I say that it was entitled to "more" protection?

A sperm and egg have potential....to become a human zygote...our first stage of development.

A zygote has realized its potential to become human. It will only continue to develop more completely over time.

I think a reasonable cut off point is conception...it's biologically distinct and knowable. It's stringent and so will prevent the problem with time based cutoffs (which are determined by last menstrual period, which isn't verifiable) being at best approximate. At least driving age is based on your birthdate, trivially verifiable.