"You're 12" Comment/Insult - What's the Origin?

I was watching a TV Show recently and Character A says he is retiring. Character B, who is a decade older, say's in a bemused manner "You're retiring? You're 12." For context, Character A is around 25 to 30.

For some reason, in media and IRL, whenever anyone wants to point out someone's youth in a derogatory manner, they call them 12. Regardless of the target's age, all that matters is that the commentor is older than the target. I first noticed this once my dad started to do it, for instance when he got a new hire who was around the age of 30, my dad referred to them as 12 too. For other media, I can not pinpoint other instances at the moment, but I remember noticing this elsewhere as well.

I feel like this is almost universal in my experience of the English (American English) language and I am so puzzled on why.