Generally speaking, children can learn a second language fairly quickly and effectively compared to adults. But I do not think there is such a particular age threshold that it is too hard to learn a language. When you are a child, everything is so fresh to you and you memorize everything as it is supposed to be. When you become an adult, you got so used to your native language, then you have to say the same thing in a different way (sound). It will not be easy, but it is not too hard to learn. It just takes some time to translate things in your mind when you express yourself.
It can be very true that it is hard to get rid of accent when you start learning a new language in an older age.