Why do people say that women in their 30s are undesirable?

If women in their 30s were so undesirable, then why are there more 30 something yr old women giving birth than 20 something yr old women? Obviously they must have boyfriends or husbands to have kids with.

https://www.healthline.com/health-news/more-women-in-their-30s-having-babies-than-20s#A-new-mother-at-30-something

https://slate.com/human-interest/2017/05/cdc-data-says-women-in-their-thirties-are-having-more-babies-than-women-in-their-twenties.html
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+1 y
If women are birthing later in life, then it means they are also likely to be marrying later in life.
Why do people say that women in their 30s are undesirable?
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