no age reqirement, but first you need to have a driver's licence. then go to a cab company and ask them to hire you. They will give you a letter of intent take that to the police station and the police will issue you a hackny licence. after you get that you can start working that day.

A note on cab driving:
I drove a taxi for almost 2 years, it's not a good job. You get paid cash evry day insted of a check, No benifits, usualy you have to pay for your gas, and you run a very high risk of being robbed. No matter what city your in there are people who think that it's ok to rob taxi drivers. your young get your ass to a mcdonalds and just work there, you still have plenty of time to find a decant job.