differen t places have different rules ex. in canada u gotta be 18 to drink U.S.A. u gatta be 21 mexico you have to be able to see over the bar table

TfL took over the running of the Public Carriage Office, who licence London’s taxis, from the Metropolitan Police. In order to obtain a licence you had to be over the age of 21 and have passed a test which required the candidate to have a vast knowledge of places in London and to be able to recite the shortest route between them. It usually took about 3 years of learning to pass the final test. So if someone had started learning at aged 18 they would just have finished at aged 21. When the private-hire industry became regulated for operators in London it was agreed that licences should only be issued to those 21 or over unless they were holders of tempory permits issued to existing private-hire drivers under a grandfather clause. Taxis and private-hire operators out of London may licence drivers under 21.

When LT operated London’s buses applicants for driving had to be over the age of 21. Now there are several operators each with their own rules but licenced by the Department of Transport. If a driver is over 18 but under 21 special restrictions apply as to where they can drive.

http://www.direct.gov.uk/en/Motoring/DriverLicensing/WhatCanYouDriveAndYourObligations/DG_4022547
http://www.direct.gov.uk/en/Motoring/Drivingforaliving/DG_182835