The Ruling on One Unable to Avoid Impurity Due to Old Age and Weakness

الجواب والله الموفق: He must avoid impurity as much as he is able, and is not required to do more than that. He should dry himself after relieving himself as much as he can, and he is not at fault regarding what comes after that on his body or clothing. It suffices him to change his soiled trousers for clean ones once a week, for example, or every five days, and to wash the soiled areas of his body if doing so causes him no harm.
The proof is Allah’s saying:
“Allah does not burden a soul beyond its capacity.” — [al-Baqarah 2:286] “So fear Allah as much as you are able.” — [al-Taghābun 64:16] “Allah intends for you ease and does not intend for you hardship.” — [al-Baqarah 2:185]
and other such verses.
The scholars of the madhhab have also said about one afflicted with incontinence of urine: it suffices him to wash his garment once every four days or so.