Are some languages more or less difficult to learn than other languages?

I don't think some languages are more or less difficult to learn than other languages, but it depends on the age. I can speak Chinese even though I am a Korean. I went to China when I was really young, and I didn't really have hard time learning Chinese. I believe I learned Chinese so quickly because I was young. My mom and dad or other adults who lived in China as long as I did or more can't speak Chinese as fluently as their children. Right now, even though I spoke fluent Chinese, I can't speak Chinese that well and when I am trying to learn again I am strugling.