Which would make sense if written as "has become disabled (from making a livelyhood) as a result of old age and infirmity" --or "has become disabled from making a livelyhood (as a farmer) due to old age and infirmity" -- or "has become unable to follow his usual profession of farming, due to disability caused by old age and infirmity".
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The disability is due to old age and infirmity.
Said disability prevents him from following his normal method of occupation, ie farming.