Why would a 27 year-old woman still act like she's in HS?

This was someone that neither me nor best friend got along with during our school years. All of the sudden, she kept posting on our school FB page about school memories, boasting about how her cool group and still talking garbage about my friend and I. She was still talking like a teenager, still acting like she's in school. A couple people replied ''grow up already'' to her.

Why would a 27 year-old woman still act like a teenager? Still be stuck on who the cool group is and who are the nerds, still be stuck at being popular?

Why would a 27 year-old woman still act like she's in HS?
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