When does life begin?

That is difficult to answer in part because the question is a bit vague. I don't know if you mean any life, human life in general or an individual person.
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Human life is a multi-generational continuous process. The gametes (egg and sperm) from which each person was formed, were alive before they merged.

Before a person’s conception, the exact sequence of genes that makes him/her unique did not exist, so that “individual” did not exist. Immediately after conception, the number of resulting individuals has not yet been determined. Twins, triplets or other results such as chimerism could occur.

When the DNA of the egg and sperm combine, there is as yet no person. There is not a microscopic person with arms, legs and a head. There is only a code for building that person, so the person does not yet exist. The construction occurs later.

Most people and religions have their own ideas of when an individual "begins". These are largely subjective.

Not until you are over 40.

States or conditions of existence previous to life are logically Impossible, therefore, a human organism waiting for life to arrive cannot exist!

At conception. Either that or when you move out of your parents house!