There seems to be gradual deterioration of language learning facilities

You know, I'm not even sure about that. It takes at least 6-8 years for most children to gain something approaching adult fluency in language. For most adults putting continuous substantial effort into learning a second language (for instance in an immersive environment), it only takes around 2 years to gain similar fluency. It may be more difficult for many adults to learn to hear and pronounce foreign phonemes thoroughly, but many adults don't have problems with that, and almost all can improve greatly with a bit of speech therapy.