Here's some info from a BBC website.
"Whether or not your vehicle can be freely driven in the Ulez depends on its year of manufacture. Broadly, these are the vehicles you can drive in London without paying the Ulez fee:
Diesel cars and vans: 2015 models and newer
Petrol cars and vans: 2006 models or newer
For motorbikes, it's 2007 and up
Buses, coaches and lorries are a little different – they're beholden to the LEZ standards
Whether or not your vehicle is compliant isn't based on some arbitrary year – it's to do with EU standards.
Every few years, as developing technology allows for greener cars, the EU updates the standards that new cars sold from that point on into the EU market must comply with.
When Ulez was first proposed for central London by the then-mayor Boris Johnson, we were up to Euro 6 standard, which covered cars from 2015. Sound familiar? That's the diesel car and van Ulez cut-off point. For petrol cars, it's Euro 4, motorbikes Euro 3 and, as mentioned above, the largest vehicles must be Euro 7."