"Life begins at?" sure. But "A life begins at" is well established.
Of course, if you ban a sufficient number of arguments that will prove your case false, you can win the day.
But this makes the case well:
The only things nec. to move from adult to oldster, from teenager > adult, from child>teen, todler>child, baby>todler, fetus to baby, embryo to fetus, or from zygote to embryo is time, nutrition, shelter, hydration & gas exchange CO2 out, O2 in).
Each of those is the name of a stage of development for some unifying "thing" that is doing the progression through those stages. That "thing" is a human being. And a human being is "a life."