> The older you are, the more effort it takes to learn new things.

My experience has been the opposite. Learning is also a skill, and at this point I am extremely accomplished at it.

> School-age kids become native speakers in terms of years with little effort

Often repeated, but doubtful. School-age children burn through incredible amounts of energy per body mass. There is almost nothing they do which is "little effort".

Probably originated because children (and adults) learn best through play, which is biologically how animals are designed to learn. But somehow "play" doesn't count as "effort"

School kids are forced to become bilingual. Every single interaction they have during the day must be in another language. The 60s year old something has most of the time limited interaction during the day with the other language. That's the main reason. A simple example, in my area we have multiple "international school" teaching only in english. I can tell you that most of the kids are far to be called bilingual in French. The other day I was in front of a group of them trying to see who would be able to order in French at the Starbuck. And this is not because they don't have French lessons, they have plenty. One of my friend is born and raised in Switzerland but only attended english schools and lived in an english speaking family, his level of French is far far from being bilingual.