Cars need to meet minimum emission standards when travelling in the Ultra Low Emission Zone (ULEZ) or the daily charge must be paid.
Petrol: Euro 4
Diesel: Euro 6
The ULEZ will be enforced based on the declared emissions of the vehicle rather than the age. However:
Information from Transport For London
Check this car on the TFL website before purchasing: https://tfl.gov.uk/modes/driving/ultra-low-emission-zone/vrm-checker-ulez