The human body isn’t made to last and when one gets old, his mind may be willing but his body will not.
I learnt that, in British English, because of the antecedent “one”, the following pronouns should be 'replaced by “one’s”. I wonder whether native speakers of British English follow this rule.
In modern English we use ‘their’ instead of ‘his’.
In this context I would say - when you get old … your mind … your body. It sounds very impersonal to keep using ‘one’ and ‘one’s’.
Alan, have one and one’s become old and old-fashioned? Don’t native users prefer them?
I would say it is used nowadays in a mocking way - often spoken in an affected way suggesting. you are some Royal figure.