Why do Americans tend to use the grade they were in to describe a time in their childhood rather than the age they were?

[6ft4]

Eg.
I knew of girls that were giving out blowjobs in the 9th grade
Last time I was under 180lbs was in the 8th grade

Why do they have a tendency to do this when they could just say the age which people universally understand
It's as if the grade they were in when things happened is etched into their memory yet despite knowing the grade they STILL can't remember what age they were in each grade in order to convert it and tell non Americans what age they were

"Yeah man I was definitely in 10th grade when that happened"
"Oh you wanna know what age was I then? hmm I'm don't remember man I must've been like 15 or 16... or wait maybe I was 14, no I think I might have been 17"

This is a reoccuring theme when I listen to Americans talking in podcasts or youtube ramblings etc
Who else has noticed this?

 

[klip11]

Im 104 years old. I was born in 1918

 

[Deleted member 18522]

Yeah, its even weirder when its homeschooled kids saying it

 

[John124]

Because when you're a kid your life is scheduled that way. You have your school grade, some people are a year older or younger than you depending on what month you were born. You have summer break and then move to the next grade. Also 1-4, 5-8, 9-12 grades are much different educational environments.

 

[OldRooster]

Even though I remember things based upon grade, I make a conscious effort to use age. I know that all I have to do is add 5 to the grade to get my age. Most people are so bad at math they can't even do that without loosing their train of thought.

 

[traveler]

easy to recall based on grade than age

 

[apocalypse]

Americans never leave school mentally

 

[Deleted member 23115]

even worse is freshman, sophomore etc but now i know how to convert

 

[Magical Apple]

Americans never leave school mentally

americans are institutionalized.

 

[Gonthar]

I knew of girls that were giving out blowjobs in the 9th grade

Where? Who?