Why do some people wait to get married or have sex in their 30s and 40s?

Like I understand it’s their choice but why so late? Do you really want your babies to have older parents? Like a five year old has 40 or 50 year old parents. One of my cousins is 41 and he’s single with no kids.
his sister is 37 and is getting married. Good for her. My neighbor is 48 and has a seven and three year old. She got married at 39.

Why do some people wait to get married or have sex in their 30s and 40s?
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