531
People born after 2000 have never seen the cosmic microwave background on their TV set.
(startrek.website)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
CRTs were fairly common until the early-mid 10s
I'd say born after 2008ish aren't likely to be familiar with them, except seeing the odd one in their grandparents bedroom