What I found fascinating is that it drops off almost completely for 27 yrs old. That leads me to conclude that 27 yrs old is simply “there” that is, it is the “Default age”. Not Young Adult. Not even adult. Just “person”.
Back in 2015, I used the phrase “acting like a [x] year old” in a series of Google searches. I tabulated the “number of search results” as a gauge of how commonly it’s used as a phrase and made a few charts.
I wanted to see who was exempt from ageism and who is affected worst.