Road tax based on emmissions . what year did it start?

My current cars are 5 & 6 years old and I'd ideally like to change both for newer cars but the chance would be a fine thing.

Did the road tax system change recently. I remember there was one concession for cars under 1.5 ltr, but I also thought some new scheme has come in where cars from a certain age get rated on their emmissions. Is this correct and if so, what year of manufacture qualifies for this.