Kids do not have the wisdom to know which bitter foods are ok and which could harm you, so they avoid them all or die. By adulthood the selection pressure is much smaller because you will have learned what is safe to eat.

"About 30 years ago, a Dutch scientist identified the chemicals that made brussels sprouts bitter. He selected seed varieties with lower levels of the bitter chemicals and bred new high-yielding varieties that tasted less bitter."

source: https://www.iowafarmbureau.com/Article/Its-not-your-imaginat...

some of it is to do with age too though. :P