It's anecdotal evidence, yes, but there are literal studies that say that it's a myth. Adults learn language better explicitly, children learn implicitly. There is no "harder" to learn based on age.

Here is one study from google:

https://www.bbc.com/future/article/20181024-the-best-age-to-learn-a-foreign-language

And there are a lot more. It's literally a myth that learning a language is harder as an adult. I do this for a living, I know what I'm talking about in that regard. You are correct that motivation and time plays a factor, however, but people choose what is important to them.