Is English difficult langauge to learn?

I think it depends on what age you start learning it. I learned it as a child so I picked it up very easily but my mother has been taking English classes for over 15+ years and while she's typically the most fluent one in her class and can understand and read in English very well she's still not very fluent in speaking it. However, she didn't start trying to learn until she was in her 30s whereas I started at 4 years old so I'm sure age makes a difference.