You have had your driving licence for the duration calculated as:
Look at your actual driver's licence first.
A table can be found on the back. This chart lists all the automobiles that you are legally permitted to drive.
See columns 10 and 11? These columns list the start and end dates of your license's validity.
The date in column 10 will often correspond to the day your driving licence was issued by the DVLA. By this time, you should have passed your driving test.
Simply subtract this date from the current date in column 10 to get the length of time you have had your driving licence.