In basic terms, ageing means an increase in scratches, dirt, dents and other such ‘wear’. Any and all of those will have an impact on the smoothness of the airframe.
As we know, laminar airflow produces less drag than turbulent airflow, therefore if possible, we would like to have as much possible laminar airflow around the body of the aircraft as possible. Scratches, dents and dirt lead to an earlier transition point and therefore a larger area of the aircraft surface will be covered with turbulent airflow, meaning more drag. Ageing will therefore increase the skin friction drag.