what' the use of English for a old man at 50's?

Whether or not English is useful depends on what you are looking for. Nowadays lots of Chinese are learning English for different purposes of their own, such as learning abroad, getting a high paid job, getting more information online, or just following the fashion etc.

For a 50-year-old man who has been learning English over 40 years, he must have known what he intended to have from it. If he got nothing from it, he could have given it up. So I don't think it's a question to such kind of man.

Concerning the "materials foreign" you mentioned above, I really find English useful when you surfing online. Many information online is written in English. You can get different opinions from differnt sides insteading of receving the same voice all day. We can see the importance from an old chinese saying "listening different voices will make things clear soon"

As for "benefit", you are a 50-year-old man, you have so many working experiences. If you can speak good English, you will definitely make a big success in your area by introducing foreign advanced tchnology, theory, or by cooperating with foreigners, or selling products to them etc.

Last but not least, 50 years old doesn't mean you are old.

对于一个年逾50而熟悉英语的老人来说，英语除了作为与他人交流的工具之外还有什么作用呢？他已学习了40年，但发现除了因为中西文化差异越来越小，可以阅读一些外来资料之外，其余毫无新意；另一方面，他也认识到这与他的私人生活毫无关系，再也不会带来财富和利益。相反，中文成为他日常生活中更为重要的部分，所以，请告诉我，英语有什么用？

for me, learning is more than using it and then paying off . it is about a way of living ,is part of life ,when you learn ,you get something ,sometimes a harvest . it brings not only money ,but also happiness and fulfillment . and language is really building a bridge to an another kind of world and civilization .
if you don`t think so ,or think it is kind of torment ,stop learning .
that is my idea ,and maybe when you desert it ,you find it is really lovely

my english is poor ,but in my opinion in our life,there are so many boring things we can't give a reason to tell others why should we do that ---

s