Should Age be from approximate fertilization date or birthing? What makes more sense?

Our age is defined when we emerge into the world as fully or mostly developed infants. Though i have always found it strange that from a cell age, we are between 7 - 9 months older based on our body development while we were still in Utero.

Does anyone think that it would be nice to use a conception date (approx.) Vs a birthing date to define when a person became a person?

I get that a lot more people would be conceived on holidays and leap years. Lol.

Should Age be from approximate fertilization date or birthing? What makes more sense?
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