As a person advances in age, the organs of the body and senses become weaker and weaker. Therefore, when a person attains fifty years of age, he should gradually get rid of karmas, whether he is a sannyasin or a householder. However, this does not mean renunciation of duty. Karma is one thing and duty is another. After fifty years of age, whether one builds a house, an ashram, a temple, a hospital or does any good deed, one can do it in the form of duty, not in the form of karma.
When a person considers himself to be a doer, whatever he does is called karma due to attachment and ego problems. When a person considers himself to be a medium, whatever he does is called duty. This difference has to be kept in mind.