Animals.

As a concept, they become less prominent in your mind as you age (exception being pets).

A 12 year old is clearly much more likely to be thinking about e.g. zebras, orca whales, tigers, etc. than a 30 year old.

Lesson in there.

i read this as specialization and a feeling of separation from the natural world coming with age in most modernized societies. Recently i came across a "complete list of animals" online. It had thousands of different animals but under the H tab humans were suspiciously missing. I also wonder if field zoologists, marine biologists, or bird watchers think about animals more then 12 year olds because that is what they specialized in or less because for them maybe the concepts have become abstract from the creatures they represent