The term is loanword adaptation.
It happens every time someone tries to use a word from a different language when speaking another. It's because every language has a different set of sounds that can be recognized as part of that language. A tongue click can be part of ordinary words in some languages in southern and eastern Africa just like any other consonant but not in others. A difference in the length of a vowel can create a difference in meaning in some languages but not in others. The presence or absence of a puff of air following a sound like p can create a difference in meaning in some languages but not in others. Native speakers of languages with such sounds and features perceive and produce them with incredible precision without ever even thinking about them, while others find it difficult.
Because of these differences, two competing goals come into play when a language borrows a word from another. One is to preserve the original form as faithfully as possible. The other is to conform to the rules of the sound system of the target language as much as possible. So when a sound used in the original word is unavailable in the target language, the closest possible sound is chosen. What you often end up is a compromise between these two forces.
Languages also differ in conditions under which each sound is allowed to occur (known as phonotactics). Languages like Japanese allow few consonant clusters. So a word like strengths is often an impossible sequence of sounds in those languages. There are two overall strategies a language can take when borrowing such a word: It can delete existing sounds or insert extra sounds. Japanese usually takes the latter approach (known as epenthesis), so e.g. strong [strɔŋ] becomes sutorongu. At the other end of the spectrum, there are languages that allow longer consonant clusters than English does. In Polish, wszczniesz [fʂt͡ʂɲɛʂ], which may be approximated by fshchnyesh in English spelling, is a totally possible word that native speakers of Polish don't find difficult to pronounce. But if you had to say fshchnyesh in the middle of an English sentence, do you think you would find it easy or the listener would easily understand what you said? I hope that gives you an idea of where the "distortion" may come from.
Do we lose the ability to pronounce some sounds as we age?
Yes, we start losing the ability to recognize fine shades of sounds when we're a few months old, and we complete this process before puberty. This is natural and essential as part of first language acquisition. If you think about it, the fact we can recognize the same word as the same word when uttered by different people, male or female, hoarse or high-pitched, in a loud room or not, having cold or not, etc., is an incredible ability that computers are still trying to catch up with. But as we start recognizing a group of similar sounds as the same sound, we also lose the potential to develop the ability to produce them distinctly in exchange.
