When you have to renew your license at 70 years old, you do not automatically continue to have the Category which allows you to drive vehicles which are between 3.5T and 7.5T on your license (C1).

Your license will be renewed without this and in order for you to continue driving vehicles of this weight, you have to fill in a form from the DVLA and have the medical declaration signed by the doctor. The details can be seen here.