CAP 744 the UK book of words on controller licensing says:
The CAA will grant an applicant an air traffic controller licence provided he:
a) holds a valid student air traffic controller licence;
b) has successfully completed a UTP;
c) has successfully completed TRUCE and been assessed as competent to handle unusual circumstances and aircraft in emergency;
d) has passed a rating examination;
e) is at least 20 years of age;
f) holds a current medical certificate of the appropriate class.
You can get a student licence if you are 18 years of age or more.